I mained Random for a good month after getting the game, I refused to "pick" any character until I had learned the best a character had to offer while playing as them was a surprise. Even now Random is still my most common choice for fun matches.
After that I mained Samus and Zero Suit Samus for a while. Samus was my main briefly in Melee when I gt tired of Young Link, and was my next choice considering Young Link's exclusion. I was determined to use them harmoniously and be good with both Samus and ZSS. In the end I found ZSS more fun to play as but I just wasn't very good with her.
After that I mained Toon Link. He was like Young Link but better (technically...) although he just didn't feel the same. I still main Young Link in Melee but Toon Link wasn't what I needed.
So I mained Sonic for a couple of months, because I like Sonic, he's awesome, and I got the feeling EVERYONE hated him. Always picking on him, making fun of him. I once thought he was popular, seems not. This made me very angry, so I vowed to get good with Sonic and win with him. Eventually though I decided that Sonic would probably be my 2nd main and I would play him for fun.
I've now settled for the time being with an old fave from Melee, Mr Game&Watch. I mained him for probably over a year in Melee a while back, and now he seemed to have improved in every way. I was very very happy to find out he made it into Brawl too
. Turns out he's 4th in the tier list too... Something I'm not too proud of, should I be?
EDIT: The strange thing is, I'm really good with Olimar, I can sometimes even beat my friends (who're better at Smash) with him, but I'm never mained him. Maybe it's because he can be a headache to use well, or because my friends generally hate fighting him. But yeah, he's awesome.