Portishead? Start anywhere, really. All records are solid work, but all of them need some time to get into. Considering the fact that Third's my favourite, try that one. But yeah, takes some time to get used to.

That being said, check out Unkle. Their album 'Psyence Fiction' is pretty good. A lot of great guest musicians too, f.e. Thom Yorke or the Beastie Boys. Also, DJ Shadow's Endtroducing is somewhat of a masterpiece.