Fretless bass is usually my favorite, especially like in Beyond Creation, Obscura, Augury, etc. It just has that awesome mournful "mwah" to it that really can give a song more character.
Just while I remember - another Peter Steele tone, which has the bass on its own at the start. Quite possibly the filthiest and most distorted bass tone I've ever heard, I thought it was a guitar when I first heard it. Magnificent.
It's great how big a part of the mix he was too - he was usually just as loud as the guitar, maybe even louder.
Also, how no-one has suggested Billy Gould is beyond me. Go to 0:45 and shit ALL the brix.