.
There are three:
*
50-135 f/2.8 with a new, ultra-fast, almost never-fail,
ring motor, ala HSM II (Sigma)
This requires the complete reinvention of SDM, which would benefit Pentax incredibly, solve some of the 'slow AF' issues, and virtually all of the reliability issues. The original 50-135 was class-leading optically, so please don't touch the optics.
Fix what's wrong with it - the motor.
A new AF module in the camera could later be introduced (or developed in tandem) with the new ring motor. Done right, Pentax leaps up a few notches in AF speed and runs with Nikon there.
*
70-200 f/4
Small, (same size as 50-135, much smaller than 60-250,) uber-sharp (ala Canon L,) very fast to focus (see new ring motor request above.)
*
135 f/2.8 Limited.
Gimme ^ .
EDIT:
No, there are four.
*
35 f/1.8
Nikon did it for $199. Charge a bit more, I don't care, but a relatively low-cost, fast D-normal
IS REQUIRED, PENTAX!!
.