Also depends on how much you want to magnify and what your subjects are...if you are looking at insects try the 100mm or even 180mm macro lenses. If you have the cash to spare the MPE65 will give you anything from x1 to x5 magnification. This is however a dedicated macro lens.
Best bet I think would be the 100mm f2.8L....very good lens, super sharp with IS.
I used the canon 50mm macro when I had the classic and was always pleased with the results. Guess it depends on what you want to shoot, a 100mm is 1/1, so you can get in real close but the DOF is amazingly thin. Have fun with the classic, they still are gold!
I had a 5d for some years before I upgraded to the MK11. I bought the 100mm macro 2.8 and it has proved a superb lens. You can get them quite reasonably now that the IS has come out, new or second hand.
It's true, the depth of field is wafer thin, but I usually shoot with a tripod anyway, and I've found that subject movement is more of a problem than camera shake, so IS wouldn't improve that. You just need to practise your technique.
Canon 5Ds with L series lenses; Apple computers, OS 10.6; Photoshop
I also recommend the 100mm focal length in a macro for it's versatility. It allows for some extra distance from your subject versus the 50mm. The 180mm gives a little more distance if you'll be shooting easily spooked insects and such, plus you get a nearly 200mm telephoto. In the 180mm length I'd go for the Sigma APO Macro 180mm F2.8 EX DG OS HSM for the optical stabilization and its f2.8 aperture versus Canon's f3.5. The 100mm is most popular by far but as always, it depends what you shoot.