Both Ulysses Siren demos are easily on par with the best non-Slayexotallideth thrash songs, with fairly technical riffwork for their time (not far removed from the traditional-infused epic-thrash of early Coroner and Destruction) and strong melodic sensibilities. They were just a song or two short of a god-tier Bay Area debut.
Forbidden's early 90's demo material (Trapped and Disillusions especially) was actually really strong, better than their studio work. There was a definite transition into more melodic and kinda grunge-ish realms, but they hadn't absorbed it to the level that they would on Distortion, and as a result they sound more like a more riff-oriented Nevermore there.
In terms of more vndergrovnd kvlt stuff, Absolute Zero's Piracy demo is really fun. Imagine The Accüsed going on an Energetic Disassembly binge and you have some idea. Violent, bouncy, crossover-infused tech-thrash.
EDIT: And fans of Adramelch should listen to Anathema's Salem Lunacy demo, which could easily be a collection of unreleased Adramelch songs, albeit the slightly less epic ones.