I know i'm going against everyone..but I actually prefer crew necks. I'm really tall and they always seem to fit whereas v-necks can look funny on me sometimes!
V necks tend to be more flattering overall. It may be because I have big boobs, but I always feel like I look boxy in a crew neck sweater if it isn't a very light/tighter material.
I vote with everyone else and say that I prefer V-necks. Don't know why, but they look better on me.