The Streamlight TLR1 is very well built, but it's a little large on a G23. If you don't mind that, it works great.
You can also go with the Glock tactical light, it's similar but without the LED bulb.
Insight makes the X2 subcompact tactical light as well, it'll be a little more compact on the weapon, but isn't as bright as the other two.
Any of those can be had for around $100. Avoid the cheaper clones - they have issues (like the battery door flying off and ejecting the batteries during firing

)