The lens I have attached more than any other... at least 80% of the time, is the Tamron 28-75mm f2.8 (continuous).
It's just an amazing lens, and it autofocuses so much better on the K-01 than it does on my K-x. It's 100% accurate on the K-01, and if I need to use manual focus for macro work that's a breeze too. (9" minimum focus distance). It's a little heavy, but I get a great zoom range for portraiture and street shooting, super sharp images, and a fast f2.8 when I need some serious bokeh or the light starts fading.
Most of the rest of the time, it has a 50mm f1.7 on it. I have barely touched the 40mm XS lens. Every now and then I'll toss the 55-300 or a superzoom on if I know I'm going someplace I need telephoto, but that's rare. The combination of the Tamron and the K-01 just roxx my soxx. I just smile whenever I pick up that camera.
Charles.