Amending my previous post (not using edit because it's upthread and sufficiently different)
If you want the best character in the game, who has very little attack lag, can survive surprisingly big attacks with proper DI/MC, with excellent reach and no bad matchups, but that everybody learns quickly how to deal with (on a basic level) because of his popularity, pick MK.
If you want a character that camps with "traditional" projectiles, pick Falco.
If you want a character that's ridiculously difficult to kill, and that has more-unique projectiles and lots of kill power, go Snake.
If you want a character that's.... Diddy Kong... pick Diddy Kong. <.<
If you want a character that is inherently designed to reward good play, pick Marth. <---This is probably the best one for newbies.
If you want a character that annoys people (other than MK), pick Wario, Olimar, or Sonic.
If you want a character that's incredibly challenging to play because it's so bad, pick anyone in the lower few tiers.
If you want a character that's incredibly challenging to play but isn't actually bad, pick Pikachu or Ice Climbers.