The most brazen and obvious rip off ever in metal was Kreator stealing riffs, vocal patterns, and even lyrics to an extent from Exodus's song Strike of the Beast, for usage on the song Total Death, off their debut. That fact is interesting to me too, considering early Kreator usually had more awesome and bad ass riffs per single song than most other speed metal bands have on an entire album. The rest of that album and Pleasure to Kill save that band from losing all credibility.
Nile - Unas Slayer of the Gods main theme is stolen from the intro/The Well of Souls off Candlemass - Nightfall. This isn't a coincidence, unless the theme originally comes from somewhere else entirely.
Savage Death, who were a pre-Morpheus Descends early 80's DM band in the vein of the other bands of that era blatantly steal the opening riff of Death's Infernal Death on the song Evil Dead from the 1985 Mass Genocide demo.
The Terrorizer song Resurrection is pretty blatant in its origin as a reworked Zombie Ritual spliced with some other stuff from the first Death album.
Suffocation is a bit liberal in their usage of the riff at 0:31 of Napalm Death's Vision Conquest for the opening riff of the song Mass Obliteration.
There is a riff on the Windham Hell song God Swallow around 1:54 that's ripped from something but I can't remember what, but it's something I've heard a lot of. That always drives me fucking nuts when I run across a riff I can't place like that. Someone listen to that and tell me what that's from.