AK-47. I can have it as completely apart as you'll ever need to take it in a matter of seconds. If you really want the trigger group off, too, I can do it in a few more seconds. Now, cleaning the AK is another story, but disassembly is fast and painless.
The Makarov is simple to field strip, but if you REALLY want to go beyond basic field stripping, it's going to take a while and be a bit of work.
However, the simplest firearm I've ever seen to get at the inner workings has to be the Tokarev pistol, hands down. Either field strip it like a 1911, or simply pop the slide stop out and quick-disassemble it that way. Then the whole firing mechanism lifts out as a complete unit, allowing it to be worked on completely outside the gun.