Portishead, generally, stands far above all other trip hop bands.
However, the Massive Attack album "Mezzanine" is, BY FAR, the best trip record ever released. I've played this album for metalheads about a dozen times, and almost each time the listened then went & purchased the record!
The atmosphere is dark, claustrophobic & stifling! It's not as "jazzy", or as "lounge" as PHead, but it's just....darker.
Another great triphop records to look for is Tricky's "Maxinquaye". It's very sensual...almost hypnotic...and dark, of course.
However, all of his other records are unlistenable shite.