I own only one macro lens, so I guess that has to be my favourite: the 100mm EF (the "regular" 2.8 USM, no L, no IS). The added bonus is that it's nice as a "normal" 100mm lens as well. The same (mutatis mutandis) is probably true for the 50mm, but since I had a 50mm prime already, I thought this was a better addition to my set. I haven't really looked at the 180mm or the very specific MP-E 65 (if 1:1 is the minimum you want).