Sloth: Rainbow Dash - As said above, her nature is to do work only when required, procrastinate, take shortcuts, and generally take a nap whenever she can.
Greed: Rarity - Rarity is known for her generosity, but it's a learned trait. She believes she deserves diamonds and jewels and all the finer things in life. She's willing to share them, of course, but only to a certain extent.
Lust: Spike - Considering he's one of the few characters that consistantly goes
Pride: Applejack - She is of the poorer pride variety. Her pride won't let her accept help, even from her own friends.
Wrath: Fluttershy - YOU WILL LOVE ME!!!!!!!
Envy: Twilight Sparkle - This one is pretty subtle, and I almost put her in greed instead (she's greedy for knowledge) but she also jealously guards her newfound friendship.
Gluttony: Pinky Pie - Okay, she doesn't even pretend. She's just lucky her naturally good metabolism has kept her from going diabetic.
I think part of each pony's struggle is to overcome her main sin, and turn that into a positive personality trait instead. Rarity turned her natural desire for goods into a means of sharing with others. Edited, Feb 22nd 2012 11:51pm by catwho