In the original Smash Bros, I mained Samus, mostly because Metroid is my favorite series.
In Melee, I mained Samus from the beginning, mostly becase I was familiar with her. But after playing as a bunch of different characters, I got really good with Ganondorf, Marth, and Shiek. After a while, I realized I was better with Ganondorf and Shiek than with Samus, so I switched my main to Ganondorf, but I never played as anybody else anymore, and lost my goodness with shiek somehow, so my secondary was Samus.
In Brawl, I'd like to utilize my current mastery of Ganondorf, but I can kinda tell they're gonna give him a new moveset. If they keep his aerial game and something similar to his down-B, I might main him again. But I'll probably switch, and if King K Rool isn't in the game, or he sucks, Snake will probably my main.
At first, I thought he'd mostly be projectile-only, but after watching his videos, he looks like his normal and smash attacks are pretty good. But obviously, since I've never played the game, and nobody has played as Snake yet, theres a chance he'll suck, or I won't do well with him.
So it's mostly a matter of time and practice with each character. But of all the announced characters so far, Snake is looking pretty good on my list of possible mains.