Well, I could just make a list like anyone and their dog, that would include all the Voigtländer Apo Lanthars, as well as the Pentax Stars and Limiteds... But we all get bored by those lenses delivering perfectly sharp and aberration free pictures! So let's get a bit more creative...
Here's my top 10 (only among the lenses I own, and in no particular order):
1. Pentax DA 18-250/3.5-6.3: the mother of all lenses. Champion for versatility and more useful than ever, now that there are DSLRs capable of delivering good quality high ISO pictures and correcting the aberrations on the spot!
2. Pentax DA 35/2.8 Macro Limited: we need at least one Limited here, don't we? To my taste, this is the best of the bunch: punchy colors, useful focal length, great contrast and macro icing on the cake!
3. Pentax DA 10-17/3.5-4.5 FE: for all the wide end needs. I prefer fish-eyes to rectilinear ultra wide angles, because I feel their circular distortion is more acceptable than the linear elongation that happens on the borders of pictures taken with rectilinear lenses.
4. Pentax F* 300/4.5: for the reach. My best telephoto and one of the most compact 300s. Works great with teleconverters too.
5. Voigtländer Nokton 58/1.4: a good quality fast lens for those low light situations. And its focal length of 58mm makes it a perfect lens for portraiture on an APS-C sensor (87mm equivalent).
6. Meyer-Optik Görlitz Trioplan 100/2.8: a fun short telephoto. Wide open it's quite dreamy with a rather unusual bokeh, but it sharpens nicely when stopped down. Good looking lens too!
7. Tamron Adaptall SP 350/5.6: a mirror telephoto for donuts lovers. Light and compact telephoto of high optical quality. Very useful close focusing ability.
8. Pentax FA 85/2.8 Soft Focus: for those pictorial shots. I love soft focus lenses and this one is a great one. Autofocus is a big plus.
9. Voigtländer 180/4: great telephoto in the pocket. A compact telephoto with superb optical quality and close focusing ability. My favorite landscape lens!
10. Helios 40-2 85/1.5: beautiful portrait lens. Although this lens is even better when used on full frame cameras, it delivers nice results for headshots on APS-C DSLRs.
Cheers!
Abbazz