I really dig Dream Theater's full-album remakes of "Master of Puppets" and "The Number of the Beast."
I may have mentioned this before, but I'm generally a huge fan of cover version tribute albums, whether it's one artist doing a bunch of other artists' songs, or an album with a bunch of songs from one artist performed by different artists. Metallica is one of the best in the business at doing that. Tesla has a double-CD tribute set that I'm fond of, Chicago finally did a big-band cover album about 22 years ago, Styx has one as well that I've listened to a few times. That's just to name a handful.