I carry two with basically what I like in lights:

1: maximum output roughly 1000 lumens, instant-on to either high setting or strobe (my choice)
Other features: It can be switched to lower lumen output, but with the push of a button, turn on to full-power strobe at any time without having to cycle through other modes

2: Single aaa battery, twist-on operation, three light levels, a glow ring around the outside edge, waterproof
Its one downside: The little tether to connect it to keys has broken, and I can't find a replacement for that part.