Although I do not yet own Brawl (I should be getting in a week or less), I will speak with my knowledge of Melee.
Allow me to join in on the Samus bandwagon. My reasoning is this:
1.) Inherent abilities: Fast and agile enough for me, with at least mediocre jumping ability. Great at a range, and somewhat powerful in close combat (though nothing on the sword-wielding fellows). Nice staying power for her destructive potential. Plus, she looks far more awesome than any other character, with the possible exception of Snake and Sheik; gotta love that suit, dude!
2.) Charge Shot: Spamming this baby is a great strategy if you know how long to charge to get the perfect power to speed ratio; amazing lateral snipage. Plus, you can roll aside to dodge attacks while charging, and then continue the charge from wherever you left off. Finally, bouncing around the screen with full charge, taking chunks off enemies' health, and then blasting them to Mushroom Kingdom Come with a point-blank facial blast, simply cannot be beat.
3.) Missile: Basically two abilities fused. One allows you to make it accelerate the farther away it gets, killing off anyone who thinks they can dodge the missile; essentially, a limited-range Charge Shot. The other lets you curve it up or down, an ability that too many people forget, and one that I use very often for that extra-special, surprise edge.
4.) Screw Attack: Just amazing. Lightning fast jump with fairly good ups and damage to knock that pesky floating Kirby right out of the sky.
5.) Grappler: Has saved me from certain doom more times than I can count... if they took my grappler away from me, I shall slay them.
6.) Aerial Maneuvering: Zoom zoom zoom! Prepare to be owned by a flying hunk of metal... with FISTS!
?.) I hear that her Final Smash, in addition to owning all, turns her into Zero Suit Samus, who sounds simply amazing, what with all the whipping and flipping and paralysis.
In close second, Marth is the final word in close-range, hit-and-run tactics, and he's just so gloriously emolicious. I hope he's still awesome in the game.