Massive Attack's Mezzanine (1998) is certainly one of my all time favorites when it comes to downtempo music. Right up there with the super chill stylings of Bonobo. I can put this album on and listen to it all the way through at just about any time. Tunes from this album have been featured in several movies. And if you've seen the TV show House you've heard the track Teardrop, which is the shows' theme song. If you don't have this album, I'd say it is a good addition to anyone's collection. It was future music when it came out and it is still so current today. Here's one of my favorite tracks:
Massive Attack is however, one of the few bands I can't recommend to see live. While I've only seen them once, they were absolutely atrocious. Their sound doesn't seem meant for a live shows but that won't stop me from listening to Mezzanine for years to come and I still think it should be part of your collection.