Stronger/More Intense:
* Furious
* Enraged
* Indignant
* Livid
* Irate
* Seething
* Wrathful
Moderate:
* Irritated
* Upset
* Frustrated
* Disgruntled
* Cross
* Peeved
Formal:
* Resentful
* Outraged
* Appalled
The best synonym to use will depend on the specific context and the level of anger you want to convey.