I can only tell you what works for me - the decision must be yours.
I always carry two tactical lights: A Sure-Fire Lumimax 4 and a Blackhawk Gladious (has multiple functions - constant on, dimming, momentary and strobe). They are both great lights for hand-held tactical purposes.
If you want to have a light attached to your G23, there are plenty of them out there to choose from. I have an Insight Technologies M3 which is a great light, but I'm not a fan of lights attached to weapons - but that's only my opinion.