Trying to come up with a post for this thread reminded me just how pretentious my opinion of music can get. Like, the Beatles are truly my favorite group of all time, but often when talking about them I'll omit them from my list of favorite bands, because, to me, the Beatles define what music is, and so including them in the list is unfair to everyone else. Trying to come up with a favorite Beatles album, I realize that it will change to one of four contenders on more or less a weeky-to-biweekly basis; between Abbey Road, The White Album, Sgt. Pepper's, and Rubber Soul. (sidenote: I've heard far too much of it already, so if you tell me that I only like Sgt. Pepper's because Rolling Stone told me to, I will fight you.)
In the interest of contributing, though, I'll name one or two of my favorites from each of a few genres.
Rap- another vote for Enter the Wu-Tang (The 36 Chambers)
Progressive- Prog Metal, I'll say Metropolis, Pt. 2: Scenes From A Memory; Prog Rock, Close to the Edge.
If I count '90s Alt. Rock as its own genre, then The Colour and the Shape and Sublime's self-titled.