I have all the Massive Attack albums and most of their b-sides, remixes etc. I consider them to be one of the finest groups of music creators around. Mezzanine is brilliant, mindbending, haunting, groovy, quietly beautiful. In my opinion it is their best album by a decent margin. It is the best album to sleep to, to study to, to #%!# to, driving with it on at night might scare you shitless though. You may listen to it once, think to yourself.. hmm this is kinda weird and boring and send it on to the bottom. But it deserves much better for sure. It's definately NOTHING like Avril Lavigne.. practically the opposite. It could take some getting into though. I would say you would be much better off to be having that CD in your collection.
As far as 100th Window.. it has the best cover of all their albums, the music on it is quite good. It hasn't had a chance to grow on me. It's really dark and spooky, def worth hearing, but cannot beat Mezzanine I don't think.