AVR assembly is one of my favorites. Compared to 6502, Z80, and x86, there is a relative plethora of registers (32 of them!) and, if your application is simple enough, you can write code that doesn't use any RAM at all. In fact, some of the smaller AVRs don't have any RAM aside from the registers.

The instruction set is organized quite nicely and most non-memory instructions (even the multiply instructions) take one cycle. Ah, the joys of RISC.