Having a good light is one those things that most of us mount but will seldom need. That said, I have tried a couple of different lights
and mounts and finally settled on a Streamlight TLR 1s. It is light, has it's own mount and also has a strobe (mine is, I think 400 lums, but they have one out now that is 600+ w/strobe).Mine is mounted on the right side forward and I can reach it for activation with my left hand trigger finger. By mounting there, it keeps my hands in proper position and the light only weighs 5 oz or so and keeps the outfit in balance plus keeping it on the right side away from my sling which is set up a couple different ways (single point or two way).
I try to make my weapons as versatile and a light as possible.