Most of my choices have more to do with liking the character, though some I do like the playstyle too.
Link: Aside from being frickin' LINK, I find his playstyle suits me, for some reason. To be fair I usually play 4 players, but I'm more consistently successful with Link than with the apparently "higher tier" characters.
Game and Watch: He's funny looking, and also tough as nails in this one.
Pokemon Trainer: I really like the idea of trying to time the switches so that you use each character's advantages as best you can--though in practice it translates to "be Charizard when your damage approaches 80% or so". Plus, Pokemon is awesome.
Samus: I actually find Samus really frustrating to play as sometimes--mostly I can't find any useful attack to use when approaching from above, and she falls too slowly to get on the ground away from opponents. But, it is Samus, who's very cool and the projectile style is an interesting one to use, so I'm trying.
Meta Knight: He's basically a ninja, nuff said.
And, not really a main, but still:
Captain Falcon: Falcon PAWNCH!--I'm not great with him but I'm getting better and I can land that Falcon Punch a little more every day--though again you can only pull it off consistently in a 4 player match if you catch someone with their pants down. However, when you catch EVERYONE with their pants down while they're all in close quarters fighting with each other...hehehehe.