HCF - Halt and Catch Fire.

I always liked rlwimi on PowerPC. It rotates the source n bits, then writes any contiguous section of bits over the corresponding bits in the destination register. This allows copying any bitfield from any position in one register into another. Basically either of these:

  out = (out & ~mask) | (in << shift & mask)
  out = (out & ~mask) | (in >> shift & mask)

Z80's EXX to swap with the shadow registers was interesting (meant for fast interrupt response so you didn't have to save registers to memory).