Sparks's Plagiarism is probably the best example. Quality is all over the map on that one. Some are bland, some are completely awful, but there are still a handful of good versions that reinvent the originals in a neat way.
I recall reading that Squeeze redid a bunch of their songs to be as close to the originals as possible cus they didn't have the rights to release the original songs. I understand that reasoning, but what a waste of time for the fans.
The Beach Boys/Brian Wilson's Smile is an obvious one (some of those songs were redone like 3 or 4 times even). We've already said pretty much everything there is to say about that though.
Blondie's album Ghosts of Download was released as disc 2 of a greatest hits compilation with the 1st disc being all remakes. That's one of the worst examples I can think of.
Ooh and Genesis' Carpet Crawlers 99. I really like that one actually. made me think a version of Genesis with Gabriel and Collins both singing could actually work pretty nicely. It kind of melds all the eras of the band together into one song.