I chose fully matured. While little bitty chicks are super cute and I can't help but fawn over them, I so appreciate when I don't have to worry about getting chick feed, checking the heat pad, supervising integration, worrying if they're really girls or not, etc.
The teenage phase is my least favorite. Chicks that were tolerant to you suddenly go fleeing if you even look at them, and they're awkward with the adults (just had one of my pullets stupidly play-challenge a hen, who nailed her several times in the back and then grabbed a beakful of feathers before letting her scurry away).
I love them when they're just hatched -- their down, barely dry beginning to fluff; their tiny chirps; the way that the wibble-wobble around; the way that they huddle up to mama to snuggle and stay warm....
Of course, the full-grown stage is great, too. The kazoo honks that the bantams make when they're getting their adult voices in are funny, too.
I love all the stages -- from egg to adult.