It depends on which alternate universe.
I have great sentimental attraction to my K1000 because it was my first "real" camera, but I shoot with the H1a and MX more than the K1000...
Objectively, the MX would be the choice in an alternate universe where I still had digital to use.
It just has a great combination of ergonomics and features.
If it were my only image capture device, and it had to be film, it would be the ZX-L (assuming I could keep it running) or the PZ1-P (if I couldn't).
There are days when I want the camera to do more of the lifting... and the ZX-L just has really nice ergonomics and a solid feature set.
Of my medium format history, It would probably be the Rollei, as everything awesome is in one little box, but a Hasse 503 maybe, for the TTL flash (I had a 500 years ago).
I've shot with both Pentax medium format cameras, but I've never owned one. I love the ergonomics of the 645, but the 67 is such an awesome beast.
And if it were truly only one, I'd stick with the 35mm cameras because they're more versatile for the way I shoot.
-Eric