I thought that the Umbara arc was one of the best story arcs in TCW (although there were some other good ones, like the Fives arc, and the Savage Opress arc from Season 3 was okay as well).
In regards to the Umbara arc, I really liked the sense of foreshadowing Episode III when Krell was explaining his motives for being evil while he was in captivity. Krell was really a jerk when he was in command, but at the same time, he was very bad-ace when he went on that clone killing-spree after he started being more evil.
I've also always been a sucker for Clone Troopers, and considering that this arc was a very clone-heavy/centric arc, I found it to be quite enjoyable (particularily the character development of those such as Rex, Fives and Tup). Also, that scene when Waxer kicks the bucket.
I'm also a sucker for clones in the sense that I really liked all of the different clone armor designs seen throughout (such as the individual 501st Troopers, AT-RT Drivers, Fives' ARC Trooper armor, and Rex's Hybrid armor).