It depends on your relationship with the person. I often say "Hiffty Biffty" to my spouse and I have a friend I always just say "Grats" to; congratulating them on surviving another year.
However, there are several, more accepted, standards:
- Happy Birthday
- Many Happy Returns
- Birthday Wishes
You can also avoid "birthday" completely by just wishing them a great day or a great year to come.
You have to ask yourself - What kind of message would you like to receive on your birthday? Personally, I would rather replace an empty "happy birthday" with a sincere message of any kind (an invitation to go out for a beer, or to the cinema, perhaps?) any day!