These days, I try to find flashlights that will accept all these replaceable battery types: (1) rechargeable NiMH, (2) rechargeable LiIon, (3) primary Lithium, and (4) Alkaline. What I currently have that meet these criteria are all AA sized lights.

My brightest easily transportable flashlight is the Nitecore P12, which runs on a single LiIon 18650 or 2xCR123. That is not really for pocket carry (although it will fit, just not comfortably), but works OK in a belt holster.

I'm sure lights that meet these criteria are available in AAA as well, but I don't have any of those personally - mine would choke on the higher voltage of the rechargeable LiIon 10440's, but meet all the other requirements. My Surefire Titan Plus is a great AAA light, but lacks the ability to accept LiIon rechargeable. Still, it's 300 lumins with a single AAA NiMH, so mighty bright for its size already.