Well there aren't really many options, really just the 100mm and the 180mm f/3.5. I would strongly recommend the 100mm f/2.8L IS, and I'm sure everyone else would agree. Very versatile lens (works great for portraits), beautiful bokeh, and the hybrid IS works incredibly well. It's just a very fun lens overall.
Now if want really, really amazing optics, the Zeiss 100mm Makro f/2 is absolutely stunning. Incredible resolving power and the separation of subject from the background almost has a 3D-like effect. But at the same time it has no IS, no AF, and it costs 2x the Canon 100mm. Now that I write all that out it is kinda hard to justify the Zeiss, but if you're flushed with cash or shoot video primarily it's a good choice.