One? Oh, boy. Way too many to limit to one.
Barnack's original set the stage form almost all other similar designs of non-rangefinder and rangefinder cameras.
Contax D that defined what the future SLR designs would be killing the Exacta and Alpa layouts.
Asahi S series, set the bar for when is a camera a hammer in disquise and likely set the stage for the split between professional level SLRs and consumer grade.
Pentax Spotmatic took the S and set the stage as the first slr with built in meter.
Tessina for probably being the funkiest 35mm camera.
Olympus F series that took the 1/2 frame format from a hobbiest format up to a near professional level.
Exacta for introducing the SLR to the masses.
Bronica ETR series as it is the only 35mm camera that can also shoot mf as far as I know.
Miranda as I think they were the first utilizing electronics such as AE.
If I had to pick only one; I'd probably have to vote for the Spotmatic as it was the first and set the standard as to layout and visuals. It set the bar for others to try to beat.