Synonym for "pissed off"

Some more possibilities:

I'm ticked off at you.

I'm teed off at you.

I'm upset with you.

You've really irked me.

I'm peeved about this.

You can say

I'm angry with you.

or

I'm annoyed at what you did.

or

What you did bothered me a great deal.

or any number of things.

By the way, if you do want to use pissed off, use at, not to, for the preposition:

I am pissed off at you.

or even just

I'm pissed at you.

I am ... with you. Not at you.

I am annoyed/ticked off/angry/pissed off with you!

Saying

I am annoyed/ticked off/angry/pissed off at you!

suggests more aggression any an ambiguity as to whether the listener well be covered in urine.

It is better to be pissed off than to be pissed on.

If you're looking for less intense feeling:

I'm irritated.

I'm bothered.

However, synonyms for 'pissed off' should really communicate the full fury of the phrase, so I'd recommend:

I'm livid.