Formal:
* Cordial: Friendly and polite
* Affable: Easy to talk to and pleasant
* Agreeable: Pleasant and easy to get along with
* Genial: Warm, friendly, and cheerful
* Benign: Kind and gentle
Informal:
* Nice: Pleasant and friendly
* Charming: Attractive and delightful
* Friendly: Showing kindness and goodwill
* Likeable: Easy to like
* Pleasant: Giving pleasure or satisfaction
The best synonym for "amiable" will depend on the specific context in which you are using it.