Most of the bands listed were already on the downside of their careers when the personnel changes were made (Ratt/Jizzy Pearl, Quiet Riot/ Paul Shortino, Warrant/Rick Steier, Skid Row/Johnny Solinger) so I don't think they can be considered the worst.
Def Leppard/Vivian Campbell - I agree that they have sucked ever since Hysteria but for some reason I just don't think it's all Vivian's fault.
Skid Row/Johnny Solinger - most people didn't even notice at this point. Even if Sebastien was still in the band the album would have flopped.
Bon Jovi/Hugh McDonald - as long as Jon and Richie are in the band they will still be Bon Jovi.
Crue/Corabi - If this album were released with a different band name it would have been huge. This is one heavy album.
VH/Cherone - Let me start by admitting I'm a huge Extreme fan and I happen to like this album. Granted, it's no VH1, 1984, or 5150 but VH had no chance after replacing Sammy. Only DLR would have worked. It's tough enough to change singers once and keep your fanbase. Has it ever happened twice???
My vote is for Poison/Richie Kotzen. Poison is a get hammered, play out of tune, party band. In other words, Poison is CC DeVille. Richie Kotzen, while a fine player, totally destroyed that vibe. It would be comparable to throwing Steve Vai in the Ramones IMHO.
Ok, now I'm off to listen to VH3 in my IPod.