What there is is a range of top quality optics on offer from Canon, Tokina, Tamron and Sigma - each with its own range of varying features, focal lengths, weights and prices. From the true macro lenses on offer there really are no truly bad options from the current offerings on the market.
The Canon 100mm L IS USM has about the same optical quality as the original 100mm macro (it is improved but only marginally since there was not that much to improve on already). Its IS and AF speed would most likely be its biggest selling points against the others on the market. Canon has always had the edge with its own macro lenses on AF speed and the 100mm L is about the top one they make - though that said AF is still somewhat slower than regular lenses.
The IS is also a bonus, though Sigma now has a 150mm and 100mm with OS and a soon to be released 180mm with OS - though I think the canon might have the edge with its hybrid IS - but I've not seen any comparative tests. Suffice to say that IS and OS in macro is very new and that its not essential for good shots (helps but isn't essential - it also weakens during macro shooting because of the increased effect of motion at magnified levels).
The Canon brand options also (if you think of upgrading your body in the future) work with the hybrid AI-servo focusing present currently in the 7D and 1DMIV (assumed also in the 5DMIII and in any advance of the midrange or higher model cameras). Again this helps, but much macro work has nearly always been done with manual focusing not auto-focus.
The 100mm L is a very solid and strong option in the market. Other options would include:
Sigma 50mm, 70mm, 105mm OS, 150mm OS, 180mm OS (also sports the widest 180mm aperture for macro on the market currently) (second hand 150mm, 180mm and 105mm options are also still found and only recently went out of production - like the original canon 100mm they are still very strong options as optically they were already top range).
Note that in macro the focal lengths have no effect on the frame of a shot at 1:1 magnification. The area captured will be the same from 35mm to 180mm - however what will change is the degree of blurring to the background. The shorter lengths giving less blur, whilst longer ones give much more. It's more noticeable at the extremes of the focal length range on offer. Note also that depth of field remains the same no matter the focal length (though the background blurring effect can make some think the shorter lengths give more depth when they don't).
Working distance also changes - longer lenses give more working distance which helps often with subjects like insects. Note that working distance is the distance from the front of the lens to the subject, whereas minimum focusing distance is from the sensor/film to the subject (which is why working distance is stated on most macro lenses).
I tend to favour the sigma 150mm myself - a bit more working range and works well with sigma teleconverters - I nearly always use a 1.4TC for a bit more working distance and a bit more magnification. I think the canon only works with teleconverters if you fit an extension tube between the two units (back of the canon pushes back too far to accept a TC). That said for pure macro work the use of an extension tube (which removes the ability to focus on further off subjects) isn't much of a limit .
Not sure about the L but the 100 f2.8 usm will work with a kenko pro 1.4x. I believe the longer the macro lens the better. Since most 1:1 macro lens are equally as sharp as the other I prefer the longer focal length versions in the 150 to 180mm range. Its easier to control the background on the longer versions and working distance is also greater.