Here’s my ranking
1)thundercatsHo (epic, many returning characters)
2)Mumm-ra lives (loved how the lunatacks got introduced and the mutants not happy about it)
3)Return to Thundera (Grune appears so that’s pretty good already)
4)Trials (didn’t really like them except for the mumm-ra and kit&kat episodes which were great)
5)Thundercubs (haven’t seen it all)
1. Thundercats Ho! - loved the action and the idea of introducing new characters (didn’t know they would take the old characters some “camera time”, which made me mad, but at the beginning I was pleased with it and it was a good arc-story).
2. Trials of Anointment - liked 3 out of 5. The good ones were Tygra, the kittens and Cheetara (this one as a plot, because it was kind of lame to do all that to end in a 5-mile race); didn’t like Panthro and Mumm-Ra’s
3. Thundercubs - it was a good one; nice to see the TCs as cubs.
4. Mumm-Ra Lives - never liked the Lunattacks, and the plot was lame IMO. Also, that’s when I realized there were too many TCs now… and Snarfer (even when I liked him more than Snarf) was also too much.
5- Return to Thundera - you know, it could have been good, but the fact of having no Pumyra and no Tygra killed it for me. They could have all returned, and then the writers just use some characters for some eps, and other characters for some others (like they perfectly did from “Thundercats Ho!” to “Last Day”), no need to leave some in Third Earth.
I like Thundercats Ho! the best. When I was a kid, I would watch “that tape” more than the others. I was fortunate to have a mom that got into the Thundercats as much as I was and would tape every episode for me and cut out the commercials.
I really liked Bengali as a kid, white tigers were my favorite animal at the time, and I still like his character design and that. Not to mention after the new Tcats were introduced, the song they would play in the background after all the TCats would show up and were kicking but was so awesome.
Mumm-Ra Lives was good too. I actually liked the Lunataks, well I liked Alluro (one of my all time favorite villains) and Chill-a, the others were a bit annoying.
Anointment Trials were ok, but I used to sometimes fast forward through those, they weren’t that exciting, but I did like Lion-O going off on Mumm-Ra during that trial.
I didn’t get to see Thundercubs and Return to Thundera as a kid, I just remember for some reason, the channel that carried TCats was dropped from our area (as I realized as I got older). I only saw up to Vultureman’s Revenge. As I got older, years later with the internet and Cartoon Network came around, I didn’t even realize I missed anything, just thought it got canceled, and finally saw them all when the DVDs came out.
Sorry to get off topic, I am new here and seem to be babbling!
Thundercubs
ThunderCats Ho!
Return to ThunDera
Lion-O’s Trials
Mumm-ra Lives
The Anointment Trials was my fav.
Mumm-Ra Lives! is my favorite, with Thundercats Ho! as a close second.
“ThunderCats, Ho!”, followed by “Mumm-Ra Lives”, “Lion-O’s Anointment Trials”, “ThunderCubs”, and “Return to Thundera”. 
1- ThunderCats Ho!
2- Mumm-Ra Lives!
3- Lion-O’s Anointment
4- ThunderCubs
5- Return to Thundera
ThunderCats Ho! is the best one by miles for me, the rest don’t even come close.
Return to Thundera was a let down, it had a good concept but the execution was bad
the episodes really dragged.
I was going to rewatch the entire series to refresh myself for the new series, however I just don’t have the time so I settled on the five parters and a few episodes. Anyways here’s my list.
1)thundercat Ho
2)Mumm-ra lives
3)Return to Thundera
4)Trials
5)Thundercubs