Majority of the macro lenses in the range of 90mm to 105mm are very good, I have not heard any bad one yet (they're all sharp), the same can be said about 50mm lens.
You can't really go wrong with any of them.
Unless you're a hard-core Macro and want the best, I believe the Lester A Dine is the one.
It is discontinued and was used mainly by the dentists for their dental works.
It is a true Macro 1:1 lens without teleconverter, built solid on a heavy side.
This lens was actually manufactured by Kino Precision Optics (no longer existed).
Kiron or Vivitar 105mm f/2.8 are other versions of the Dine,they are all identicle.
Here is a discussion about this lens:
The only drawback of this lens is MF, but for macro works you don't need AF anyway.
I own top caliber Nikon zooms w/ AF, yes they are convenience but I like MF.
MF will make you aware of where you are and what you're doing, you have to move around to compose the picture.
You will gradually master the MF and become a better photographer (I'm learning the technique as well).