Kinda depends on a lot of factors, to be honest, even as a Sonic diehard, they're all kind of mediocre in their own way. Either the animation is cheap, the story is too formulaic, or the concept is so abstract that it doesn't even feel connected to Sonic anymore (in Underground's case, all three lol....). This is coming from someone who really enjoys AoStH and SATAM, but mostly for external reasons (the art style, lore, nostalgia, etc).
I guess the "best" one would be Sonic X, there's still a lot of anime/cartoon campiness in the show, but it does have its moments, and if you just want to see Sonic and the gang doing stuff with decent animation, then it can give you that, occasionally surprising you too.
Or you could watch Tamers12345, the REAL best Sonic show.