I'd say there are three great go-to pop albums, and one go-to experimental.
Transformer is the furthest extension of the Loaded-era Lou Reed, and would have been spectacular as the fifth official (sixth, really) Velvet Underground album. Berlin is a darker, more bizarre beast, a concept album based on a great, disturbing lyric and some more nuanced musicianship. Street Hassle, his 1977 album, has an incredible title track that fills most of the album and other songs that are nearly as good and stretches to some great heights. Marco mentioned it perhaps in joking, but Metal Machine Music is one of the great early noise albums, full of symphonies of guitar squall and feedback filling four sides of vinyl and around 60 minutes of release. If you like noise music, you'll probably love Metal Machine Music, and I'm a huge fan.
DO NOT listen to LuLu, the release he did with Metallica. it's god-awful. Same with The Raven, his album-length interpretation of the Poe poem.