I thought the last 20 episodes of UA were pretty fun.
I thought I'd make a list will try to keep arc episodes out, did not like double or nothing or perfect girlfriend
Simian Says
Flame Keepers Circle for being a decent Vilgax episode
Mother of all vreedles
Enemy of my frenemy
Inspector 13
Couples Retreat
Catch A falling Star
Eggman COmeth
Greetings from techadon
AS for the early UA
Hero Time and Reflected Glory were great,
HIt em WHere They Live and Video games all fun
When it came to the actual arcs they had to grow on me, I think Aggregor Amalgam form was awesome too bad there wasn't a big finale for him. The Map Of INfinity arc showed how much Kevin has grown and they redo the mutant Kevin arc so we get more insight into his origins in the OS and why he was willing to help Ben in Ben 10 Returns not through lines but behavior he mellowed as a teen, I think those OV flashbacks in the rooters arc ruin this, Kevin also wasn't THAT insane in the OS when he was a mutant
As for me there is no best season of UA it all merges and rounds out Charmcaster, Anodite Gwen, Darkstar the Techadons, Jennifer Nocturne , Captain Nemesis , Vulkanus, JT and Cash all get good episodes before OV negates or not use them at all
MIddle UA is okay but that's where I thought it became aimless the jimmy jones episode couldve been better, the ben julie breakup could've been made a great arc, Ben 10000 return is boring