I look forward to you picks Lynch...it was hard for me so the tie breaker was Which band I liked the most along with the best cover art.
The hardest part for me is distinguishing between cover art that I love and the actual album inside the sleeve. I mean, it should have no effect, but for me it is. I could put a half dozen Iron Maiden albums alone on my list, always loved their stuff, and that's not including the covers on the singles. Iced Earth has some really cool album covers as well. Same with a number of other 80's and 90's metal bands. Then I look at an album cover like Back in Black. Simple yet perfect. Not the least bit complicated, yet to me, incredibly classic.
I have my list down to 25 right now. Hopefully I don't think of anything else that I forgot and end up jacking it back up to 35. Might have to start flipping coins.