The headsculpt isn't exactly make or break for me, but it can be the crowning glory on an already good figure. I was looking at my Animated figures there recently, and a lot of them have horrible headsculpts. Flat, off-model, or just generally weak. Sentinel Prime on the other hand has a phenomenal (...and hilarious) headsculpt. It really elevates an otherwise average toy. Some of the best headsculpts can be found on the old Diaclone TF molds. Very sharp,robust detailing. I think some of them hold up better than their Classics updates. They really were top notch work, both then and now. They could've got away with a lot less. It was one of the things that set Transformers ahead of Gobots, back in the day.
I don't really value head sculpts that much in my transformers. I usually put the most weight on kibble volume, a character I'm fond of, cool factor, etc... If a figure has a cool head sculpt, that's great, but it doesn't make or break a figure for me. That said, one of the main reasons I've never bought a movie Megatron is because the heads (and molds) are atrocious. Cybertron Repugnus and Undermine are my picks for absolute worst head mold ever. I still own Repugnus, though.
