Photoshop. I've become so accustomed over the years to the tools/interface, that it's actually faster than pen/paper. Setting it up with common UI elements to drop in quickly makes mockups a breeze.
Photoshop for me as well, because I can then easily transition from the wireframe to the high-fidelity mockup. I much prefer it when the evolution is a continuous process, rather than being interrupted by switching software.
I used to feel that way, but when I began using separate framing software I was pleasantly surprised by the results. Having to start the design phase from scratch in a new program freed me from slavish adherence to the frames and allowed designs to progress further. Horses for courses though - I'm sure everyone has their own take.