General sense:
* Likeable
* Charming
* Pleasant
* Friendly
* Nice
* Agreeable
More specific nuances:
* Easygoing (if "simpático" refers to someone who is relaxed and not easily offended)
* Warm (if it refers to someone who is approachable and welcoming)
* Affable (if it emphasizes the person's easygoing nature and ability to converse)
Ultimately, the best English translation depends on the specific situation and the intended meaning.