Haha, that's funny because I really couldnt stand Envy. But that was because of his attitdude, his/its death in FMAB was done perfectly though.
In my opinion, it would have to be pride..l mean it's confidence. Although it is excessive confidence, and I can see how it can be detrimental; but it's also very important to have confidence in yourself.
I guess all the of sins are actually good for you but with consequences in the long run. Remember envy means something very different to what it did back when the 7 sins were created. After all, Envy IS ugly, but it's inherintly human and that's so weird to me that this is something that comes naturally to me and literally cannot be fought. You can stop greed, wrath and gluttony with a lot of self control but no amount of assurance really gets rid of jealousy.
Aw man I LOVED Envy the Homonculi. Best character in the whole thing along with Greedling & Hohenheim. Actually i've just noticed I have a trend for sassy, b*tchy characters who are filled with jealousy. (Kabuto Yakushi, Envy, Loki, Scar from Lion King) that says a LOT about my psyche huh? And oh man, that DEATH. That whole episode is art. The way Envy just keeps on pushing Mustang wayyy too far to the extent of even impersonating Hughe's wife had me on the edge of my seat. And then Mustang went batsh*t insane. The sad thing is, I really feel like in Envy's last moments I was able to somehow relate to him, and that really hurt to be honest. The fact that this resentfulness and condescending attitude was actually forced and all he really was in the end was jealous of Elric and his friends- and he couldn't accept that. Beautiful.