Plain back or center pleat. Although I absolutely hate a box pleat on normal dress shirts, I somewhat like it on an OCBD. It's useless to me functionally, but it's in keeping with tradition. Which a lot of aspects of clothing are. No shirred sleeves/back, they're terribly hideous. A shirred sleeve when overdone (I've seen a really pronounced Neapolitan one, which is no surprise since they seem to enjoy exaggeration) looks like a woman's dress shirt and is a terrible look.