For a 6 volt light I like the Surefire Scout (M600) light with the incandesent bulb housing (like this much better than the LED that comes standard on the Scout....and it's cheaper) and clicker tail cap on a LaRue Tactical mount. Nice small lightweight package:
http://www.coloradoshooting.org/usmc03%20new%20fg3s.jpg
For situations when more light is desired (ie. clearing long hall ways, schools, industrial buildings, alleys, large outside areas, etc). I prefer the 961C 9 volt with the clicker tail cap and LaRue mount:
http://www.coloradoshooting.org/usmc03%206920sd4s.jpg
I have the "clicker" tail caps on all of my lights, as I have seen too many accidental light discharges with tape switches. And tape switches don't give you the option of "constant on" if you need to perform tasks with your support hand (ie. opening doors, picking up items, grabbing / holding a suspect, etc). Another benefit is that they are cheaper than a tape switch as well.