Toy

Best instruction set:
http://www.kernighan.com/toysim.html

loop
  load B
  print
  store A
  load C
  store B
  add A
  store C
  goto loop
A 0
B 0
C 1

This is kind of EXPERT! Don't see why Kernighan would write something like it, though. (The documentation on the site is almost fucking useless, too, I actually had to read the interpreter's code.)