Choose whoever you enjoy playing most, and choose whoever caters to your kind of playstyle (this goes hand in hand.)
If you like Sonic, play Sonic. If you like characters that move fast than any of the spacies would do you well along with Diddy, Captain Falcon, Sheik, Marth, Roy, TL, Lucario, Donkey Kong (not super fast on his feet, but the combo potential is monstrous), Meta Knight, and porbably more that I'm missing but that covers a good chunk of the cast. Pikachu?
Anyway, experiment with the cast! Go into Training mode, try out their moves, and attempt to do a few combos. Although it's not a real match, it can give you a rough idea of what the character is capable of and how you would use them (like how a Samus combos might put the opponent out of her aerial range, but allows her to follow up with her charged neutral b. Whereas someone like Falco is able to throw whole combo strings together just using aerial attacks. Obviously lots of characters can do this, but my point is the difference in how they all use their specials and regulars together which effects the playstyle of that character.)
This is turning into a ramble x) Experiment, find who feels good man, and you're in there.
