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I own the 100mm L IS, Sigma 150mm f2.8 OS, and the MP-E 65mm macro lenses. For insects and ultra-small stuff, nothing comes close to the MP-E, for versatility, nothing comes close to the 100L, since the AF is ultra-fast and precise, and the hybrid IS is really helpful, both in nat light macros and in general/close-up shooting. For skittish, "long range" insects, like butterflies and dragonflies, the Sigma 150mm is nice, since it takes a teleconverter very well (unlike the 100mm, see above), and has a long working distance.
a micro-moth with the MP-E
a robber fly with the 100L, in natural light
a bumble-bee with the 150mm, in natural light
a dragon fly with the 150mm + 1.4x = 210mm:
The 100L and 150mm works very well as portrait lenses, too. The MP-E does not