Sonic Underground was too much of a mix of SatAM and AoSTH. Eggman was an idiot, there was wackyness, and the two mutts (That were enemies of Sonic) were basically flesh-and-blood versions of Scratch and Grounder, who were actually a tad smarter, and very much funnier.
SatAM was a great show, and showed a lot of emotion. It also had great art and music, however, without Sonic and Tails, a fan (at first glance) would have no idea it was related to Sonic. Too much SEGASonic is a bad thing, but not enough works the same way.
Sonic X would've gotten my vote, if it had just slightly more depth. The art is fantastic (Being the same as the games' today), and the writing is good, but the humor aspect isn't quite as much as there as in another one of the Sonic cartoons. Plus, they edited out too much of the good stuff for the US/UK edit. Meh. >_<
Thus, and I am not afriad to say it, but Adventures of Sonic the Hedgehog
gets my vote. The art may have been somewhat bad, and the lip-synch was occasioanlly off (But hey, it was only 1993-1996, give DiC a break ^_^). It had that hillarious element that the other three cartoons simply did not live up to, with an incredibly mix of characters. All of them were smart in SOME way (Okay, mabey not Grounder... ^_^;), but they all had a stupid element that made them funny, especially Eggman. I loved it when he had his temper tantrums XD. It also had the most of the game series in it (Aside from Sonic X), and that makes for something that Sonic Underground and SatAM especially didn't have. I just grew up with the cartoon when it aired on UPN38, and it was part of my early childhoood that made part of me today. AoSTH gets my vote.
And, BTW, here is my order of votes:
1. Adventures of Sonic the hedgehog
2. Sonic X
3. SatAM
4. Sonic Underground
(And PS, the reason I didn't put in the Sonic Anime, was because I was talking about cartoons that actually aired for a while, not a feature-length movie ^_~)
Someday I'll find the AoSTH mini-series "Quest for the Chaos Emeralds" on the internet...