This is just my two cents worth from my personal experience, based off of who I'm good with, who I've seen others be good with, and who the cpu is good with. It's not definitive, but it's also not random guesswork. =D (I'm also just addressing 1-on-1 battles, because it gets really screwy to try to address full brawls also)
Top Tier
Marth is the ONLY member of the Top Tier. (I only ever use him in competitive fights, as it's just mean to beat up on new players who are still getting used to Brawl. They won't even have time to test out all their attacks before you finish them off). It's almost as easy as breathing to use his aerials, and chaining them is pretty much your opponent's death warrant. His improved side-B is a heck of a lot better in melee and can actually be useful. His down air takes some getting used to in order to spike consistently, but it's a pretty awesome edgeguard with little danger if you jump toward the stage right when you use it.
Summary? With one opponent to focus on, I can't see why you wouldn't win with Marth. Intelligent use of techniques combined with a healthy dose of mindgames and trickery should seal your opponent's fate.
High Tier
Pikachu. Very quick, surprisingly powerful, and a real pain to get kills on with his recovery. It's also really funny to triple thunder someone off the top.
Olimar. His range is blasphemous with all his pikmin out. His aerials somehow dominate. As long as you don't forget to bring your pikmin out after losing a stock (it will happen...) you're set to win.
Toon Link. Fast, still has Link's double f-smash, his down air is nothing short of murderous if used correctly, and good recovery. For extra intimidation, change to his black costume for demon link.
Wolf. His side-smash is a dash attack, his dash attack is an up-smash, and his up-smash launches like crazy. He's heavy enough not to get pummeled like his SF adversaries, his side-B going somewhat up opens up a lot of possibilities, and the end of his recovery comes with a slash that can actually kill someone who was careless enough to get hit by it.
Ike. He definitely doesn't have Marth's speed, but his power is absolutely obscene. He kills most characters at damages they shouldn't even go off the stage at. It takes a good amount of prediction skills to land most of his attacks (assuming your opponent is a functioning human being) and he's pretty open after some of them, but his power is just so astronomical that you probably only need to land 3-4 attacks to kill someone.
Mid/Low Tier
(Instead of listing all of these, I'll just address why some of these aren't High Tier, because I'm assuming people will ask those questions.)
Sheik. It might be just be, but she just doesn't seem as cheap as she was in melee.
Snake. He's far too slow, at least for me. In the time it takes him to USE his f-smash, some characters could finish him off. While he's excellent fun in group matches, from using his mine to owning with his final smash, he's far too easy to predict and run circles around in a 1-on-1 situation.
Captain Falcon. He could very well end up in the top tier, but he seems way too spazzy and way too light to be effective. He's up for interpretation.
Diddy Kong. Lots of people praise Diddy, but I'm just not seeing it. To be honest, there are probably Diddy mains who are very good with him, but like I said, this is based off of what I've personally experienced, and I haven't seen him be good.
Low Tier (read: sucky)
Mr. G&W. A mediocre smash player could predict everything G&W could do, and barring straight 9's on his side-B, there's not much you can do with him.
R.O.B. I don't understand why he's even here. He has everything you wouldn't want in a brawler, from being easy to edgeguard to being slow without the power to make up for it.
Feel free to critisize. =P