You weren't very specific, but the Streamlight Protac 90 is an amazing little backup light.
From the spec.sheet:
"AA alkaline/lithium output
– High: 125 lumens; 2,150 candela; 93m beam; runs 2.75 hours (alk); 5.5 hours (lith)
– Low: 40 lumens; 700 candela; 53m beam; runs 8.25 hours (alk); 15 hours (lith)"
It has zero parasitic drain, with a lithium AA you can store that for years and have it ready immediately.
Technically it has two modes, but if you weren't aware of this fact, you could use it as a single mode light, without ever realising the second mode.