I think one or two of you might be surprised by these results:
http://www.nnplus.de/macro/Macro100E.html
The Tamron 90mm is very highly thought of with one at least one well known pro describing it as one of the all time great macro lenses. The Tamron 180, Sigma 180 and Canon 180 macros were tested by AP magazine, and the results were akin to those in the link above.