Who says happy, smiling fiends died with Planescape...?
The Pit Fiend on the cover of the FCII looks like he's clutching onto that sword like a four year old would their teddy bear. And what happened to Strahd's face, he looks like he had a head transplant with Herman Munster.
However I love the Cityscape and the Shattered Gates covers and the remainder are at least... pretty decent. Dark Matter's not bad either.
Just a nitpick though about the gelugon - why would an ice devil have frosty breath? It doesn't even breath, never mind being a cold creature, I'm thinking it's breath would be the same temperature as the air around it.