My first suggestion would be the Crimson Trace (CTC) Railmaster series...They have a few variations and after trying them out I like them.
Another option would be the Lasermax Uni-Max lasers...They work pretty well and I've used a modified green version with good results.
Of course, the above assumes you REALLY want an under-barrel/rail-mounted laser. I have and prefer the CTC Lasergrips as the best option out there right now.
I have used the Streamlight TLR-2 on an M&P 9mm and it worked very well. I like being able to control the light and laser together and with the Contour switch this gives other systems a run for their money (yes...Surefire makes the X400 with DG option, and it's awesome, but it's also WAY more expensive than a TLR-2 with contour switch). I also put it on my HK USP 45 thinking it would work well...It did not... The recoil from the 45 made it impossible to maintain consistency in holding down the switch, and since there is no contour switch option for the HK (as there would be for your M&P) I took it off.