Depends on the working voltage of the laser, size, required current draw etc.
I like Lithium's for most applications, higher voltage from a single cell, higher energy density and higher discharge rate. Just need to watch out for cheap Chinese cells - although I haven't had any issues myself, but I'm not using them in any lasers with crazy current draws like a NUBM44 or some such.
I am using NiMh D-type from ANSMANN. It is the best choice due to the size.
But in small lasers i prefer Li-Pol 18650. But, as far as i know NiMh batteries are much safier than any Li-pol