Formal:
* Aunt - This is the most common and formal spelling.
Informal:
* Auntie
* Aunty
* Anti
* Anty
* Aunti
Other variations:
* Aunte (archaic)
* Auntie-M (when referring to a maternal aunt)
* Auntie-P (when referring to a paternal aunt)
The best spelling will depend on your personal preference and the context.