There is no best macro lens for the camera. The choice of macro lens should be based on what you intend to shoot, how much weight you are willing to carry, whether you will use a tripod etc. I suggest you take a little time and read up on these things to make the best decision.
It's generally agreed that all modern macro lenses provide sharp images. The most basic choice is focal length, which relates to how far you can be from your subject.
If you have any specific questions as you are doing your research, I am sure you will get some help here.
I own a Sigma 150mm 2.8 as well, and I love it, but the other posters are correct - there is no 'best' macro lens; it all depends on what you want to shoot, and in my case, how much weight you can deal with. For instance, the 150mm, for me, is quite heavy and I find I need to use a tripod in order to get great shots. If you prefer to hand hold, then a lighter weight macro lens may be better suited for you. Again, it all depends on what you want and your desired outcome.
Chris has a great idea - scroll thru the macro forum. Then do some research to see which lens would be best for you. Another idea - maybe there's a camera shop near where you live that provides lens rentals? If so, that may be a way for you to test the waters, so to speak, before committing your $$.
Part of the reason so many are saying "there is no best" is to do with focal length and speed. A 60mm is great for sharp focus but tiny critters are going to get REAL uncomfortable with you getting that close. A 105mm or 150mm would be better to maintain a respectable distance.
The other side of the equation is speed. Any movement in the macro world is magnified under higher zoom. The faster the lens the better if your subject is moving (VR comes in handy here). If you are doing product shots on stationary objects (e.g. jewelry, etc) this is a relatively non-issue.
So you see, deciding on what you want to shoot is key to picking a lens.