What's a good iphone lens to get?

As a gift for a friend who likes to take photos I want to get one of those lenses that you can attach to your iphone to enhance it but I really don't know which to get! Which would you find more useful - fisheye, wide lense, macro? I don't want to spend too much, can anyone point me in the right direction of which brand or model I should be looking at.cheers!

Added (1). Oh and it's for an iphone 4s

Ask your friend what they want.

Lenses that attach to iPhones are not inexpensive.

http://www.iprolens.com/examples.php

There's a lens that combines all three of the types of lenses you want to buy for your friend!
The lens is called "Olloclip." If you do decide to purchase this however, make sure you buy the one made for the iPhone 4/4S, not the one made for the iPhone 5.