Hello Tom,
basicly you can use any Sigma lens on hand.
The 18-200 works good at the long end (i.e. 70-200)
I got the best results with the Sigma 150mm Macro so far.
It also depends on what you are going for,if you are at home and can arange
your set-up, the distance does not matter you can go close and use shorter
lenses.
If you are outside and go for insects, you may stay a bit away,and therefor
a longer lense gives you more distance.
There is no "best lense" for all (macro)purposes.
If you plan serious macrowork,you should look for a 105 or 150 Sigma macro lense in the long term.
Regards
Uwe