^^ I'd say Post is most important, but lets not split hairs here
There are premixed shakes out there for pre and post workout, the biggest problems with them are they are one size fits all, so you may be taking in more/less then you actually need, and they are all expensive IMO. Mixing your own is the best way to do it, this way if start to put on a little chub, you can back off on carbs, ect. Many of the premixed shakes also contain a decent amount of fat, which you dont want in your pre or post w/o shake.