My favourite characters (that are in Brawl and/or Melee) are: Sonic, Luigi, Diddy Kong, Donkey Kong, Mewtwo, Mario, Yoshi, Wario...
In Melee I first played Mario, just because I mostly played Mario games and loved them, and so I liked Mario. After I unlocked Luigi I nearly always used him. Sometimes, but not often, DK, and very rarely Mario. When I got Mewtwo I used to play it sometimes... well, maybe often ^^ but it didn't even come near to my Luigi.. ^^ Luigi's my favourite character in Melee, but now.. since some months I started playing DK very often. I guess 5 times DK, 1 time a other character or something like that. Now Luigi is... a little boring /: I guess the whole game isn't what it used to be. It's still great, but the more I played it the more I don't want to play it. (well, I mostly play with my little brother and some computers, but if I play competitively I can't get enough Melee XD)
Anyway: Sonic is my favourite video game character
so I'll use him in Brawl. Doesn't matter how bad he sucks (but it looks like he doesn't ^^).
My favourite list of all video game characters is probably like this:
Sonic, Knuckles, Luigi, Mario, Diddy, ...
I like Diddy very much because of DKC2. It is such an awesome game... (oh the memories... >_>)
So I'll use those characters very often because I like them. And because I like many characters, it's not very likely that all of them suck and that I can stick to at least one of them if I want to play competitively =)
Now there are characters like Falco... I never really played Star Fox games. (Well, some years ago I played Lylatwars on the N64 and beat it. Sometimes play against friends, and once I played Star Fox Assault (I think) with some friends, but I mostly liked Slippy ^^)
Anyway, I like Falcos playing style and his attacks, and that's because I play him very often.
Tiers don't really say who I'll play with.
It's more like this:
-> I like many character and play as them very often
-> I'll get good with the characters I play often...
-> I'll use the characters I'm very used to competitively
Now if they are VERY LOW tier, like Pichu, I'd probably won't use them often anyway... just because they suck so much. If Luigi wasn't my style and would hurt himself or something like that I probably wouldn't play as him very often... >_> Hmm... and I like Bowser, and use him sometimes, but only just for fun... I'm not really used to him and don't like him THAT much that I'll really use him... but if he'd be my favourite character or something, I'd still play as him and probably try him competitively ^^
And as you can see by my name I use mostly Luigi. Even competitively.
I just really like his personality (it's funny^^) and his playing style... I love how he slides so much.. lol.
I liked if even before I knew how to wavedash... I always ran to my enemies, and during the running i charged up a smash attack and slide to them and if they're in reach I hit them.
So one can probably say that, for me, the first and most important thing is that I like the character. The second important thing is his playing style and if he sucks or not...