I like boxer briefs mostly. Nothing heavy or baggy, or made of that rough, slightly crinckly fabric some boxers come in.
As for anything more unusual, I think i'd be quite turned on by guys in ladies undies, but that's only academic because i don't really enjoy it in porn.
Thongs, g-strings, jock straps and pouches. Not really interested. I think it takes a certain athelitic figure to pull those off and they aren't flattering for the average bloke, me included.