The Streamlight TLR-7A Flex is the best balance of size, output, and cost. If you are running a mid-size pistol (Glock 19 or similar), it’s hard to beat using the TLR-7. There are more powerful options out there, like the X300U, but they are much longer and heavier.
The TLR7-A Flex puts out a very nice cone of light rated at 500 lumens, the one CR123a battery can be changed from the front, and the front lens can be rotated as a lock-out feature to prevent accidental discharge. Nobody like accidental discharges.
The rear buttons can activate the light from either side with a sure click. If you press and hold the switch, it will act as a momentary switch and turn off when you release it.