You can pretty much take Mew in almost any direction you want to. On top of being able to learn every TM/HM, it can also learn any Move Tutor move. With a base 100 in every stat, it gives you some flexibility as to whether you want to go offense or defense and even on top of that physical, special, or mixed.
I know this sounds like my general advice for everything, but you should take a look at the team around Mew and see what you want it to do. If you need offense, you can go offense. If you need defense, you can do that way. Or there are other gimmicks you can do as well.
Here is a list of all of the moves that Mew can have in Gen 6. Keep in mind that all of the "Transfer Only Moves" are only available to you if it already has it, since they come from other generation games.
http://serebii.net/pokedex-xy/151.shtml