Casual:
* Blown away: Means someone is very impressed, often surprised.
* Amazed: Shows strong admiration.
* In awe: Indicates a sense of wonder and respect.
* Thrilled: Expresses great excitement and pleasure.
* Floored: Means someone is shocked and impressed.
Informal:
* Stoked: Means someone is very excited and enthusiastic.
* Pumped: Indicates a feeling of energy and excitement.
* Mind blown: Shows someone is completely surprised and impressed.
* Wowed: Expresses a strong feeling of wonder and admiration.
* Gobsmacked: A British term for being shocked and amazed.
* Smitten: Expresses being captivated or charmed.
The best slang term to use will depend on the specific context and the speaker's intended meaning.