Actually, you guys might be surprised, but Link and Samus are the best. Here's why:
I invented this combo. I call it the ubercombo. What you do is play normally until one of your opponents gets 50% damage or more.
Link or Samus then uses their superior grab range to grab the victim, and Luigi comes in for a quick UP-B dragonpunch to knock them out. I'm serious, this works. I invented this a while back, but actually tried to test it out today. I got a random friend to sign up via an irc chat that I frequent, and we totally beat another one of my random friends with it. And get this: this worked even though my friend accidentally chose the wrong team (all three of us were in a free for all basically). His link would grab my opponent's wolf, and my luigi would KO wolf while doing like 5% worth of collateral damage to Link.
And the next game we actually tried it with teams, the game ended something like this:
Link: 9 lives
Luigi: 9 lives
Lucario: pwned
:D
Of course, this might not be fool proof. Link/Samus has to be a good grabber, or the combo is pointless. And of course, we didn't do a 4 way (never enough friends online >.>), so it's possible that by the time Luigi realizes that it's his turn to do a smash, the victim's teammate might beat up link/samus out of the hold
Well, good luck! Oh, and variations involve Jigglypuff's rest (doesn't work very well anymore in brawl), and captain falcon/ganondorf's reverse punches.
Oh, and if you guys like my strategy and actually use it, please make some youtube vids and credit me for them. Lol.
Who knows; this might be the best way to win contests, and may make luigi the most popular character in all of brawl; heck he might even be a "top tier".
Lol, I agree with Tr4Q though :D