I might go as far as saying that MS Paint for Windows Vista is the my favorite program period. It had every feature I could want (excluding transparency) and nothing else. The interface was quick and intuitive. I think it was pretty much perfect
MSPaint does actually have transparency (at least the XP version has)! The "secondary color" desides which color is to be transparent, if you move stuff and have transparency enabled (in the "selection" options).
So it's even sort of possible to emulate layers by having multiple MSPaints open.