the G2 triggers are usually the most widely-used due to their price. they can usually be had for about 30 bucks or so, and most places that sell AK parts will sell the G2 fire control groups.
the red star arms adjustable is the other one that immediately pops into my head, but it is a bit more costly at around 75 bucks per set. i've been told it's very very very nice, but since none of my AK's are meant to be match-grade rifles, i've stuck with the G2 and been very satisfied.