For general resentment:
* Animosity: A strong feeling of dislike or hostility.
* Aversion: A strong dislike or disinclination.
* Bitter: Marked by strong and often angry feelings of displeasure.
* Indignation: Anger aroused by something unjust or unfair.
For resentment rooted in a sense of unfairness or injustice:
* Injustice: The state of being unfair or wrong.
* Grievance: A cause for complaint or protest.
* Rankle: To cause irritation or resentment.
For resentment directed towards a specific person or group:
* Vindictiveness: A desire for revenge.
* Spite: A feeling of ill will or malice.
The best word will depend on the context and the specific kind of resentment you want to emphasize.