The first thing I thought of when I saw this thread was Ico. Hands down one of the worst covers ever. I wouldn't be surprised if it's part of the reason it sold so poorly. It completely undermines the games artistic brilliance to package it in something quite the opposite.
I agree with this to an extent. They weren't so bad in the SNES era, and I really liked the limited edition case art for XII.
The original Mega Man got pretty well ganked in cover art. I don't have time to track down a picture right now but I'm sure someone will oblige for those who haven't seen it.