The best character I found was definitly Lisa. She could beat any character in any circuit.
It's very easy with her to stop a special (O) move that takes any amount of time before it can be used by just playing her sax at least two times. This is especially useful for Flanders.
Her bite special does also can do a lot of damage.
One of the hardest characters I found to beat was Apu when he does his Kung Fu or whatever it is, and you pretty much have to figure out your own strategy for him (I liked jumping and using O to knock him down) Kang can be really easy with her if you know how to handle him.
Because Lisa is very quick she can just go right up to him and continually beat him up with her square move.