Lots of medium format cameras are really cool, but I am gonna have to nominate the following as EXTRA COOL, based on both style and substance:
1. Fuji GX680 series. The deformed love child of the Mamiya RZ67 and the Linhof Technika...ON STEROIDS. The Batmobile of medium format cameras. (The ugly, bulbous, Michael Keaton Batmobile, not the bitchin' '60's one.) It terms of looks, it's high-tech, modern, and very late-80's/early-90s. However, it's hideousness is made COOL by the fact that it is such a kick-butt camera system. Not only is it an SLR that shoots 6x8 frames, but it has front movements! The only thing that could make it much better in my book would be if it shot 6x9 (and didn't need that damned battery). Oh well; I'll settle. It's one of the most capable cameras ever made that operates as quickly and seamlessly as it does. It's like having an RZ that is just that much better in the studio. (As if RZs are not good enough already.)
2. Mamiya C series. Interchangeable lens TLRs. As an every day user, there is not much cooler in the world of medium format IMHO. All the coolness of using a TLR, plus the benefits of cool things you normally don't get with a TLR (i.e. a 7-lens system). And the older ones with the silver lenses and gray "M" vinyl covering look SO COOL. I have shot so much with my C33 and C220 because they are absolute joys to use, and I LOOK SO COOL using them!
3. Mamiya Press series. Giant, multi-format rangefinders with interchangeable lenses. If only they were SLRs, they'd be the best cameras ever made. Oh well. They are still pretty darned cool, and have that same awesome Mamiya styling that I mentioned above. I must say that the old Fuji 6x9 RFs are probably better users, but they don't even approach the Press system in terms of cool factor.
RB Graflexes and Arca-Swiss Reflexes
Linhof 6x9 view cameras
6x9 press/field cameras like Linhof, Horseman, and Graphics
Rolleiflex Wide Angle