Three come to mind; Rush's Fly By Night (the owl flying towards you), REO Speedwagon's You can Tune a Piano but You Can't Tune a Fish (tuning fork inside fish) and whichever one is the ice cube on fire.
While I do like Roger Dean's designs I am particularly fond of the work of Storm Thorgerson and Hipgnosis. They have created some of the most iconic album covers ever.
Ever since I saw the Saturday's Headlines cover I loved it. It was just unique and cool and awesome. I'm kind of a girl for putting it but you guys asked and I answered!
If you want to see it : http://www.theprophetblog.net/the-saturdays-unveil-headlines-album-cover-and-tracklisting