General synonyms:
* Assertive
* Pushy
* Forceful
* Dominant
* Confrontational
* Hostile
* Belligerent
* Combative
* Attacking
More specific synonyms depending on the context:
* In business: Competitive, ambitious, driven, tenacious, relentless
* In sports: Ruthless, tenacious, determined, fearless
* In personal relationships: Dominating, overbearing, controlling, pushy
Less negative synonyms:
* Enthusiastic
* Energetic
* Passionate
* Strong-willed
The best synonym for "aggressive" will depend on the specific situation and the tone you want to convey.