Favorite Macro Lens

Canon 100mm, Sigma 105mm, Canon 180L, Sigma 150, MP-E65.... Take your pick, they are all awesome lenses with great glass.

The MP-E65 can't really be grouped with the rest of those lenses, it's a highly specialized macro lens that can't be used for any other photography. The *maximum* working distance is about five inches...

Most 100mmish macros are great.
If I had to do it again, I'd probably go for the Tamron 90mm. Great price, if you can get it with a rebate, great 6 yr warranty, comes with a hood and a bag. According to tests like photozone.de, a bit sharper than the canon version, but not a big deal as we're splitting hairs. The Canon is a IF so that's nice, but it's more expensive and does not come with a hood. The front element is close to the edge of the lens so you risk bumping it into subjects doing macro work.

I've read an extensive German test (can't find it again now) that had the Sigma and Tamron slightly (minor differences, really) sharper than the Canon lens, but that was only at large apertures. At small apertures (typical for macro use) there was no significant difference, if anything the Canon was slightly sharper and contrastier. I don't think you can go wrong with any of the macro lenses as far as optical quality goes, you have to weigh in other characteristics to pick your favourite.

The Tokina 100mm macro is no slouch either. In a recent Shutterbug magazine comparison, it bested the Tamron and the Sigma.