The LX might be the greatest Pentax 35mm camera ever, however it was not necessarily a significant Pentax... Significant may be a combination of market innovation and sales numbers.
The Asahiflex as the first instant return SLR
The Spotmatic as the first TTL SLR
(I'll argue the ES as an early successful auto exposure SLR)
(I'll also argure the F as the platform for which SMC lenses were introduced)
K1000 as the VW Bug of cameras
MX as the culmination of SLR miniaturization
ME as the SLR as Point n Shoot of the day (these seem to have sold by the boatload)
The 110 slr because it was a 110 slr
The 6x7
The digital 645 as doing something with medium format price/performance
I'm also discounting the ME-F as a market fail, and without my fanboy glasses, I can't really say much else stands out as different and significant in the marketplace, as Pentax lost its earlier positioning in the market. Still made/makes excellent cameras, but these are Pentaxian distillations/refinements of other people's innovations.
Am I forgetting some camera?