If you want to be able to pocket carry it, and use it with a handgun, it has to be small enough for the pocket, but large enough not to fumble it with the gun. If you are using it with a gun, it also has to be able to go to "high" each, and every time, when you want it to. You do NOT want to be in some low, moonlight or flashy mode when you need it to shoot. IMO, the best light to use with a handgun, for this reason, is a single level light. This doesn't make it as useful for other uses as a multilevel light, but it's a matter of priorities. It also has to be bomb-proof.
That said, there are a few two or mulit-level lights (there may be others) that I have experience with that I would be comforrtable using as a light with a handgun, though I fortunately have very infrequent need of those capabilites.
One is the HDS/RA. A 120 Tactical would be ideal, as it comes on to "high," but you can easily click to a lower output level if needed. There are brigther HDS lights, but the 120 is really pretty bright, and is the most economical of the HDS offerings.
The other light I'd recommed is a SureFire L1, the latest version with the 15 / 65 lumen output. The 65 lumen high is really closer to 100. This light will let you press the tailcap firmly to go into high, or you can depress lightly to go into the 15 lumen mode. You can also twist the talcap for constant on low, or constant on high.
And, while I haven't any experience wtih one, the E1B should also work well. 110 lumens on high, 5 on low. Comes on in high mode. Not ideal if you are going to be turning the light on and off while moving around, because the second time you turn it on, you will go into low mode, if it has only been off for what...2 seconds or less? That can be distracting and ... unfortunate. But if you aren't going to be doing that, and it's just going to come out and go "on" when the gun is drawn, it's probably a good choice. I had an E2DL, same UI, and it seemed to work well enough that way.