What paint should I use to customise an iPhone for case?

I want to paint the Incredible Hulk on an iPhone case for my older brother for Christmas, but what paint should I use?
I've used acrylics before to paint my PS2, but it scratches off so easily and this is something I want to last.
Any suggestions or other options I can use?

There are 3 main types of paint… Water, acrylic, and oil paint.
water paint will just wash off,
acrylic is a mix between water and oil paint,
oil paint is the best. It is more expensive than the other two but oil doesn't mix well with water there for its water proff once it drys. It also takes a very long time to dry.don't make it to thick or it really can take weeks to dry.thin coat might still take a few days.
since your painting a phone or case it mostlikly will still get scratched because it is an item that gets moved around so often. For example if he put it in his pocket with his keys. If you want the picture to stay a very long time/ hopefully forever you should go to Walmart back in the photo area and buy a cosimized photo case. You get to pick whatever photo you want and they print it on a case. It looks like its made to be on the case it's very cool. Maybe you can just print off a picture of hulk from the Internet or something or if you really want you could paint the picture of hulk then take a picture with your phone then take the picture of your painting to them to get it printed off.
hope I could help.