I plan to watch some old epic battles in anticipation for the new show but since there are so many I need some help.
My faves are in no order
Mumm-ra vs the demolisher
the demolisher vs lion’o
Jackelman vs lion’o in “the mountain”
berserkers attack on the berbil island in “thundercatsho”
Lion’o vs King Arthur/Mumm-ra
The Thundercats versus the Technopede in “Turmegar The Tuska” is pretty awesome.
I liked the one between Monkian and Tygra in “Monkian’s Bargain”.
Jaga vs. Grune
The “rumble in the pyramid” at the end of the fifth Anointment Trial is worth a look too.
My fave is Jaga vs Grune. It was epic, something about the two astral forms of Grune and Jaga up in the sky and larger than life sent chills through me. Not to mention the coolness of Lion-O throwing him the sword of omens and later giving him his strength.
The Sword of Omens Vs Excalibur, it was epic to me when I was seven years old and it still holds up today.
The ones that stand out to me:
Thundarian Fleet Vs. The Plundarrian Armada from “Exodus” = This interstellar battle was so radically different from the rest of the show that it easily stands out. Also, we get to see death portrayed in the show as many Thundarian ships are coldly destroyed by Slythe’s forces. One could argue that Slythe & The Mutants accomplished more in this one episode than Mumm-Ra & any of the other villains did in the entire series just based on the body count. And then of course once they board the Flagship, we get to see Tygra, Panthro, Cheetara, & The Thunderkittens show off their stuff, which is nice.
Mumm-Ra Vs. Lion-O from “The Unholy Alliance” = As a kid, I legit feared for Lion-O’s life as I knew he was no match for Mumm-Ra, and early on, he wasn’t. And they got pretty creative here with using a tree trunk as a weapon, and mixing Lion-O’s fear with his willingness to prove himself etc.
Tygra Vs. Monkian from “The Slaves of Castle Plun-Darr” = We got to see Monkian use that awesome shield while also getting to see Tygra do what he does best.
The Break in to Cat’s Lair from “Pumm-Ra” = After dealing with the Mutants, we get to see the TCats try to break into Cat’s Lair after Pumm-Ra has locked them out. This leads to an exciting bit of scenes where they are trying to get in, particularly of Panthro scaling the wall while a giant Moth creature (Mothra?) tries to knock him off.
Jaga Vs. Grune from “The Ghost Warrior” = This is actually my personal favorite in the series. The two ghostly entities going at it above Cat’s Lair…it was a pretty epic encounter.
The Thundercats Vs. The Inflamer from “All That Glitters” = You get to see the T-Cats try to get the Sword of Omens back as an erupting volcano is threatening them. It was pretty crazy.
Pantrho Vs. “Panthro” from “Spitting Image” = This creepy episode ended with Panthro dueling with his double as a forest burns around them, leaving Lion-O not knowing which one to help. This was really the only true Panthro centric episode in the series, so it was cool to have him get this climax and it was visually striking as well.
The Thundercats Vs. The War-Bot from “Return to Thundera” = Want to see Cat’s Lair get TORE UP, then check this out! Also, Lion-O coming into make the save and say “Sword of Omens, Give Me Power Beyond Power!” to force the War-Bot back was pretty epic.
Lion-O Vs. Safari Joe from “Safari Joe” = My jaw dropped when the T-Cat Sword Call didn’t work as the Thundrainium cages kept the T-Cats subdued. Safari Joe was also a very different kind of foe for Lion-O and it was great to see a Gun Vs. Sword battle done fairly effectively in addition to working the battle around hunting prey rather than typical come in and all go “BOOM!” kind of stuff.
Lion-O Vs. Ratar-O from “Feliner: Part 2” = This epic one on one clash between the Lord of the Thundercats & The Great Pludarrian Warlord showed off why Ratar-O should have been used more frequently in the series.
The Demolisher Vs. Mumm-Ra from “The Demolisher” = This bad guy came in and whooped up on Mumm-Ra…you know he’s serious business!
Lion-O Vs. The Demolisher from “The Demolisher” = Basically the whole episode was just Demolisher trying to beat up stuff, and his showdown with Lion-O was pretty epic.
Lion-O Vs. Jackalman from “Jackalman’s Rebellion” = Who knew the cowardly Jackal could put up a fight against the Lord of the Thundercats?
The Thundercats Vs. The Technopede from “Turmagar the Tuska” = This fight was really cool, and unlike most of what you see in the series. I loved how it took the combined team work of all the Cats to get the job done here to take down this gargantuan mechanical monstrosity.
Lion-O Vs. “King Arthur” from “Excalibur” = This was just awesome. I loved the medieval set up. I loved the joust. I loved them just throwing the rulebook out and getting into to good ol’ Sword clashing, and in one of the strangest things to ever happen in the series, we got to see The Sword of Omens & Excalibur leave their owner’s hands and fight each other! And what really puts this fight over the top is the fact that Mumm-Ra actually wins.
The Battle at the Onyx Pyramid from “The Transfer” = This was nuts! We got Mumm-Ra on top of the Pyramid holding a nuclear reactor. We have The Thundercats following him up there in the Thundertank and then jumping out to scale the slopes of the Pyramid on foot and stand their ground to attack Mumm-Ra from all angles. And for flavor, we got Slythe and Vultureman attacking both forces in the Flying Machine. This was just wacky and the way I’m describing it is nowhere near as cool as it looked.
Lion-O Vs. Jackalman from “The Mountain” = Way to go Jackalman! Gets on here again as he actually manages to outsmart and defeat Lion-O (as they go over a waterfall!) to take the Sword of Omens from him. Again, I never knew that yellow dog of a cowardly cat had it in him.
Super Powered Vultureman’s Battles with Mumm-Ra & The Thundercats from “The Super Power Potion” = The major appeal of this episode is to see Vultureman just totally kick butt throughout. He outmatches Panthro & Tygra individually, and probably the biggest shocker, he practically humiliates Mumm-Ra, who is reduced to begging for his life. And then it all comes down to an epic climax against Lion-O, who is also over powered.
Lion-O Vs. Panthro from “The Trial of Strength” = What a way to start the trials, huh? You have to go fight Panthro on a rocky mountain side. Good luck with that, ha ha. Watching Lion-O match power with Pantrho was pretty cool though.
Lion-O Vs. Cheetara from “The Trial of Speed” = OK, not really a fight, but a race, and it was one of those moments where I was just like “HOW?! HOW IS LION-O GOING TO WIN?!?!” and to me, it was definitely the toughest of the T-Cat trials, at least on paper.
Lion-O Vs. Mumm-Ra from “The Trial of Evil” = No Sword of Omens, No Thundercats, No Jaga…Lion-O is Dead Cat Walking! At least that’s what I was thinking when I first saw this episode. The whole thing just seems really hopeless for Lion-O, but thankfully, they didn’t use Mumm-Ra’s reflection as the means to defeat him. Lion-O persevered and eventually found his opening to exploit Mumm-Ra’s secret weakness.
Lion-O Vs. “Lion-O” from “Fond Memories” = There were actually a lot of cool little fights in this one as Lion-O deals with Ratar-O & Safari Joe again, but the best was at the end where Mumm-Ra assumed the image of Lion-O and the two do battle. It was an interesting visual if nothing else.
The Battle at Hook Mountain “Thundercats Ho!: Parts 2-3” = There is a really LONG sequence spent at Hook Mountain as the Thundercats do battle with The Mutants. This includes involvement from Snowman & Snowmeow, The Thundertank, The Fist Pounder, & eventually The Rat-Star, all combining for one exciting series of events and moments.
Lion-O Vs. TugMug from “Mumm-Ra Lives Part 2” = “So this is the legendary Sword of Omens, huh? Ha-Ha-Ha” and with that TugMug became my favorite Lunatac. Not only did we get to see his awesome physical powers, but we got to see him snap the legendary blade with no effort at all. Unfortunately, he probably peaked a little too soon as he never quite recaptured this level of awesomeness, but it was still a pretty shocking moment.
The Thundercats Vs. The Lunatacs from “Mumm-Ra Lives Part 3” = TugMug had already showed off in Part 2, but here we get to see a little bit more of what the Lunatacs are capable of, namely Alluro & The Lunattacker being a match for The Thundertank. Lynx-O & Snarf turning the power of the Tower of Omens loose on Sky Tomb was pretty awesome as well.
Pantrho, Cheetara, & Snarf Vs. Ben Gali, Pumyra, & Snarfer from “Catfight” = Um, yeah, AWESOME!!! Thundercats Vs. Thundercats. Old Guard Vs. New Arrivals. It was pretty sweet, especially seeing the Thundertank do battle with The Thunderstrike.
The New Thundercats Vs. The Berserkers from “Together We Stand” = This was a nice epic battle between mostly new characters, and I thought it was a bit of a spotlight moment for Ben-Gali as well.
The Thundercats Vs. The Mutants from “Ravage Island” = Here, I just got a kick out of watching The Mutants play with the T-Cats’ infected minds and defeat them with ease. Anybody who watched the show will tell you that didn’t happen much.
Young Jaga Vs. Ratilla from “Thundercubs: Part 1” = Its always cool to go back into history, and to see Jaga do battle with an ancestor of Ratar-O was pretty freakin epic! It served mainly as exposition for The Sword of Plundarr & The Treasure of Thundera, but it was still very nice and different from what we had been seeing up to that point.
Lion-O Vs. Mumm-Ra from “Thundercubs: Part 5” = Another epic encounter between the two mortal enemies. To see that giant Super Powered Mumm-Ra with his Sword of Plundarr go up against Lion-O was actually pretty tense.
Lion-O Vs. Mumm-Ra from “The Thunderscope” = People tend to forget about this episode, but Mumm-Ra actually kills Lion-O! He breaks the sword and just leaves him for dead! Of course Jaga has to interfere and fix it all, but I’d say this is the best Lion-O/Mumm-Ra fight that nobody remembers.
Hachiman Vs. Mumm-Ra from “The Jade Dragon” = AWESOME F’N FIGHT!!! It wacky as hell, but it is one of the most insane things they ever did on this show. Hachiman duels with the giant statues, the cauldron is now a telaportation device, and the Sword of Plundarr is tested against The Thundercutter!
Lion-O Vs. Mumm-Ra from “The Last Day” = Had to put this on there. Sure it ended with Mumm-Ra running out of time, but it was awesome to see a Godzilla sized Mumm-Ra just rip through Third Earth with ease and duel with Lion-O infront of Cat’s Lair as the sun sets in the background. It was just really well done.
Lion-O Vs. Pyron from “The Book of Omens” = The Mumm-Ra fight here was only Greco Roman wrestling, for some reason, but at least we got to see Lion-O contend with this armored hell beast! It was pretty cool!
I too like those battles where the TCs worked as a team, like the one Pkell mentioned against the Technopede. For that, I’d have to also mention:
. TCs vs. Wolfrat in “Wolfrat”: it was great to see the TCs in disadvantage, working as a team against this giant mechanic rat.
TCs vs Mutants in “Mycrits”: I know some people didn’t like this ep, but for me it was great to see the TCs fighting for their lives with no weapons and no Lion-O either, just tricks and ability. It would have been better, IMO, to have them solve the problems alone, and Lion.O gettinmg only by the end of the ep, but alas, he was the hero after all.
The one in “Trouble With Time”, where Lion-O and Cheetara distract the flying water snakes while Panthro sneaks an aged, withered Tygra into the Geyser of Life. Yes, it’s not a battle per se, but it’s an enjoyable conflict none the less. 
I like when the Thunderkittens were avoiding the Black Widow Shark. Saw that one on TV recently.