To be honest, I never really thought “trip-hop” was a genre, more of a lazy catch-all label used by the music press. Do Massive Attack & Portishead really have that much in common (other than Bristol)?
Ooh, lots of good stuff mentioned in this thread. A couple more:
- Thievery Corporation, “Richest Man in Babylon”
- Hooverphonic, “A New Stereophonic Sound Spectacular” and “Blue Wonder Power Milk”
- Supreme Beings of Leisure, self-titled
… and some other stuff you might like, if you like Massive Attack:
- Arling & Cameron, “We Are A&C” and “All-In”
- Death in Vegas, “Scorpio Rising” and “The Contino Sessions”
- Black Box Recorder, “Passionoia”
Well, maybe, but that would just completely undermine my rant, wouldn’t it? 
I’m going to claim a tenuous connection in that both Portishead and Massive Attack (and most other bands lumped in the trip hop genre) combine hip hop rhythms with dark, cinematic soundscapes.
Sorry to derail your rant. Although if you want something you can rant about to your heart’s content, try listening to “Barbie Girl” by Aqua. An aural atrocitry.
True, they have a bit in common, they are both very atmospheric. But Massive Attack were heavily influnced by Jamaican Dub, which Portishead doesn’t have a trace of. And Portishead are heavily Jazz influenced, unlike Massive Attack.