The Tamron 90mm is a classic, but both the Sigma 70mm and Pentax 100mm have better build. The Sigma 70mm is supposed to be the best optically of the Sigma macro line (per Sigma Web site) and probably on a par with the other two (there is no photozone tests for the Sigma 70 with a decent sensor). A lot depends on the desired focal length. I have the Sigma 50/2.8 EX DG Macro and love the build and optical performance. Working distance at 1:1 is another matter however. A few mm of focal length goes a long way in terms of buying more working distance. There is a penalty in terms of length and weight to be considered as well. There is also the matter of WR. I live in a wet part of the world and the utility of a WR macro lens for field work is indisputable from my perspective.
My advice? Order all three and keep the one you like? Or maybe make a trip to the local photo store and check out the comparative build quality and handling first hand. One thing to also consider is that the Pentax lens lacks an aperture ring. This limits its usefulness with film bodies and/or bellows and extension tubes.
Steve
BTW...back when I bought my Sigma 50, I really wanted the 70 but price at that time was $250 more...
The Tamron did not make the cut due to build and bulk despite aggressive pricing.