Where do you plan on carrying the flashlight? If it's in the pocket, a 18650 cell light is going to be on the larger size and not something I'd consider pocket carry. A RCR123/CR123 light would be much more compact and comfortable to carry. I currently own a 4Sevens Quark 123 and a 4Sevens Preon (and a bunch of larger lights). I find myself carrying the Preon more often simply because it is so small. Not exactly a defense carry, but it's very convenient.
These are the CR123 lights that I would take a look at. If I was in the market for a small compact tactical light, the Olight M10 would be my first choice.
Olight S10 Baton
Olight M10 Baton
One more thing to think about. If you're planning on using Li-Ion batteries, make sure you buy a quality PROTECTED battery (AW) and a quality charger.