In the UK I reckon, Muddy Fox Courier and Specialised Rock Hopper.
These brought mountain biking to the masses.
I don't really follow the principal that the 'first' or most exotic are necessarily the most influential. Mercedes-Benz built the first cars; but as far as influence goes, the Ford Model T and VW Beetle have without a doubt had a greater influence on car use.
Same goes with mountain bikes, sure Breezers are cool, but no one would have heard of them if it wasn't for the likes of Specialized and Kona.
.........and sus and full sus??
Would never have happened if there hadn't been rigid first and I don't really think it effected the popularity of the sport that much, but I could just be biased.
Modern: Cotic Solaris | Cotic Escapade
Retro: 1988 MF Explorer | 1991 DB Apex | 1992 DB Response | 1994 GT Zaskar LE
WANTED: RS Mag 21 with 1/18" unthreaded steerer | USE Stealth forks or spare parts