Bontrager OR (pre "race" / "racelite" – circa 89 /90) :
Bonding, riveting, bisected stays, proper gusseting, chainsuck plate, horizontal dropouts and bombproof steel frame. Engineering genius.
Same of the Bontrager comp fork;…so much attention to detail. The privateer wasn't a patch (sorry Ton).
Others:
Bushido – for showing what might be possible
Mantis XCR – Composite possibilities
Cunningham / Bradbury (not Answer!) Manitou – Alloy possibilities (shame the latter liked to crack)
RC100 – holistic / integrated design approach
Merlin Mountain – Ti simplicity
Mtn Goat Deluxe – cycling art
Mtn Cycles San Andreas – With Suspenders & pro-stops – yum
Yeti C-26 – Tomac never looked better
Foes XC-T – suspension for people who don't like suspension 😉
I could make a very long list 🙂