In addition to my Canon 100/2.8 Macro that I voted for, I also have the MP-E 65 1-5x Macro, although of course it's not a prime. I have yet to get stuck in with the MP-E 65 in ernest, but so far it's been fun to play with when used in conjunction with a Canon MR-14EX Ring Lite on my 20D. A friend came over last night. I handed him my camera with MP-E 65 and ring lite on (I set everything beforehand) and he took a shot of some newspaper print no problem at all at 5x handheld. He has a Nikon system, but started talking about Canon. His fiancee may not want him to be getting any more GAS* ideas from me. Don't we all suffer from GAS to some degree or we wouldn't be doing what we do, right?
* Gear Acquisition Syndrome
The MP-E most certanly is a prime - why do you say it is not???