Formal:
* Top-notch: implies high quality and standards.
* First-rate: suggests something of the highest quality and excellence.
* Superb: emphasizes exceptional quality and skill.
Informal:
* Awesome: a very popular and versatile term for anything great.
* Rad: a more laid-back and cool way to say "awesome."
* Sick: an informal slang term meaning "amazing" or "really good" (often used for music or clothing).
* Epic: used for something that is grand, impressive, or memorable.
* Wicked: a British slang term meaning "excellent" or "very good."
* Killer: a slang term suggesting something is incredibly good.
* Brilliant: a more intellectual way to say "excellent."
* Fantastic: a more playful and enthusiastic way to say "excellent."
* Splendid: a more old-fashioned and formal way to say "excellent."
More specific to a certain area:
* Tight: used in some communities to express approval of something cool or well-done.
* Dope: a slang term meaning "excellent" or "amazing," particularly used in hip-hop culture.
The best slang term for you will depend on your context and audience.