The Canon EF 100mm f2, 100mm f2.8 USM Macro and 85mm f1.8 lenses have a good reputation for being sharp. There also seems to be a lot of quality variation from one lens to another so if you can try the lens first that is the best way.
I have the 100mm f2.8 USM Macro, and it is good (not excellent) wide-open. To get near perfect corner - corner performance requires stopping to f5. Here is a shot wide open with my modded 300D:
http://www.pbase.com/terrylovejoy/image/36529489
If you do a search for posts by 'Pertti' on the yahoo digital-astro list you will see some detailed lens tests.
Terry