This is my favorite:
D26 XM-L2 LED Diving Flashlight
They are under $60 on eBay, with no battery or charger. I liked my first one so much, I bought a second one just to have a spare. They use one battery (a 26650 rechargeable), so they aren't too long to be comfortable in a soft mount on the back of my hand. They last a long time on one battery. They have 4 brightness modes plus a strobe mode. And they are bright. And well made. And they have a push button on/off, which can be locked to prevent accidental on or off. And they have a colored LED to indicate battery charge status.
Those two lights replaced my second favorite:
DGX 600
DGX 600 Handheld Light | Dive Gear Express®
$50 with an okay battery and a charger. It's about as bright as the Xtar. They have the same single LED emitter. But, it's a smaller battery (and smaller overall size, which is good). And it only has one brightness level, so you can't set it lower to make it last longer. And it's twist on/off, which takes two hands.
I also have one of these:
EDI-T cheap Chinese light
5000LM Ultrabright T6 26650 LED Diving Flashlight Torch Light Waterproof 45MX
$25. It's okay, but it's not any brighter than the Xtar - just has a little bit broader beam. But, the light head is so much bigger I find it annoying. But, it IS only $25...
With good batteries, the DGX600 will easily last an hour and a half. If I use it much on a dive, I put in a fresh battery before a second dive. The Xtar light takes a bigger battery and I feel comfortable using it two dives on the same battery. Especially since the Xtar has 4 different brightness levels. I would have no problem using it on the same battery for 2 night dives (i.e. it's on for the WHOLE dive). I would just use it in Medium brightness (out of Low, Med, Hi, and Turbo). Medium is still plenty bright for almost any need during a night dive.