Welp, I'm the one person going for Sonic then....'obvious choice', I know, but it's really no secret that I more-or-less collect Sonic games unless they're difficult to find(Knuckles Chaotix), or just aren't worth getting/don't-have-the-money-to-waste-on-them(Sonic Riders 2). SO that'd have to be at least one reason.
The other reason is that people constantly say he's so fast that he's tough to master, so I kind of want to at least TRY. I've heard he's weak in attack, but if he's fast enough, I think it might make for an interesting battle, dodging, jumping in for a small attack, dodging, etc., but just be fast enough to make it happen quick.
Otherwise I tend to main as Samus, especially in the original game where she just seemed much faster and more control-responsive(she seems so much slower(in response time) in Melee for some reason, but it's probably just me).
As for the others and why I didn't pick them(oh sure, I'll use them ALL at some point, but as far as potential to be a 'main' goes)...
Metaknight: WAY too often, I keep hearing of how he seems either overpowered or maybe just spammy. I really like that he's in, but I think he's too popular for my liking.
Pit: I specifically hated that game, although I have some sentiment for it since it's been long since abandonned. However, I see him as an air-based Link with a little more emphasis on bow-fighting, probably with light attacks, so I'm not TOO interested.
Snake: Snake can be cool, but he's kind of made different for this game. I always think it's funny when an otherwise serious game/character does 'stupid/funny' stuff and maintains a serious demeanor, but that doesn't help too much here. Anyway, the main difference seems to be making him explosives-centric, and I'm usually not that type of fighter.
Wario: Again, I see him as a quirky, slow-type character, and I rarely fight as slow-types(I hardly EVER play as Bowser or Ganondorf). He's definately got some cool things, and I like how he animates so jerky like he would on the Gameboy on in his Microgames, but as of now I don't see him as being something I could naturally 'pilot' to my liking.
Ike: Not really into FE much, but I guess it's just the thought of another slow/heavy character and another sword-user. It's possible I might like him, but again he's reported as being overpowered, and I don't really care to be thought of as the person who's doing that just to try to get ahead..
Pokemon Trainer: I think the idea is great, but the execution just seems like I'm playing with three somewhat-bland fighters rather than one really unique fighter.
Diddy Kong: I could possibly like him, but again I see him as a light-fighter and one that I'm not extremely fond of exactly(he's alright, but I think it was from DKC2 where I'd prefer to be Dixie and I'd get irritated when she'd die and I had to play as him(since he didn't have that floating mechanic that could potentially save you from a fall). Otherwise he looks pretty decent, but I'm just not sure yet.
Lucas: Sorry, but I'll never play Mother 3 since it's Japanese only(I mean, I'd either have to play with a script on-screen which I dislike doing, or actually put effort into my self-studies

). As well.... I know this comes off as awful, but I've never liked Earthbound too much. But MORE than that.....just looking at this from a Smash Brothers side of things, I've never liked playing as Ness at all. No offense, but I just don't care for it...and since Lucas is like an extension of Ness, I'm hesitant to even try to play as him(I will, but he'll either be a 'let's get this out of the way early' thing or I'll just put him off until I've played with everyone else).
King Dedede: Another slow character....that pretty much sums up why I don't *think* I'll main with him, and probably never 'sub'(?) as him either. I think it's cool that he's in, but I'm not 100% satisfied with his moveset.
Captain Olimar: Again, I think it's great that he's in..... but from what I'm seeing, he just seems to be an otherwise-defenseless/offenseless character who, including the Pikmin as items, can only battle using items. It seems like he's either too simple, or too hard to keep track of(in thought of getting more Pikmin and knowing which ones you have and which to use). I think he's cool and all, and I'll definately use him a few times I'm sure, but probably not too often.