I'm partial to 1284P and Bobuino's. (thanks to RetroLefty for the name). Have found the 2 hardware serial ports, extra IO without needing to add shift registers & such, and large SRAM to be great for projects.
Prior to that, I favor Duemilanove also for simple fooling around projects to check out timing of software stuff. Also to add high current shift registers for clocks & displays.