It's all over the place, I find, but I typically go for a close-up of the model's best feature/s:

for the pretty ones, I zoom-in on the face (a 3/4 shot).

for stunning features (tattoos, piercings, muscles, bum etc) a close-up of it.

if the set is not of the best quality, I'll go for the best hardcore image - yes, I know many will not go there, but I do!