All of them are pretty decent. The weakest in my eyes is probably Dilettante, but it is still okay.
Dragonborn - Breath Weapon rocks.Low damage doesn't even matter. Free minion killing is great, so is free multi marking for fighters.
Dwarf - Second Wind use for minor action. Looks deceptively weak but I find it to be fantastic.
Eladrin - Teleport. Pretty good in general. Let you reach something you wouldn't or couldn't have, or just plain escape a bad situation ( for that round, maybe more, hehe )
Elf - The reroll is pretty solid. Nothing irks you more then missing a crucial attack.
Half Elf - Requires an at will that is compatible with your primary class stats to be kind of effective. For instance there really aren't good ones for a warlock in my eyes. An at-will on a 1/enc timer is pretty weak in general.
Halfling - Second Chance, pretty sweet, not game breaking, but still nice to make someone possibly miss you.
Human - A free at will is pretty good as long as you have the stat to support it. Nuff said.
Tiefling - A small bonus to hit, and a decent bonus to damage ( assuming a decent cha) is okay, but not one of my favorites.