What you actually say, though, should be based off your relationship with each person. If you are not very close, "Happy birthday" may be the most appropriate. If you are very close, you can perhaps play off some inside joke or personal tie to express the sentiment. "Happy birthday" is the standard well-wish, but you can vary it in a great many ways to say the same thing.
Remember, the reason you are saying this is to wish your friend well on their day of celebration, so it should be something well suited to them.