Formal:
* 极好 (jí hǎo) - This literally means "extremely good" and is a very formal way to express something is fantastic.
* 完美 (wán měi) - Meaning "perfect", this is also a very formal and positive expression.
Informal:
* 太棒了 (tài bàng le) - This is a very common and casual way to say "fantastic" or "awesome".
* 太酷了 (tài kù le) - This means "so cool" and is used to express excitement or admiration.
* 真不错 (zhēn bù cuò) - This is a more neutral way to say "pretty good" or "not bad" but can still convey the meaning of "fantastic" in certain situations.
Other options:
* 了不起 (liǎo bù qǐ) - This means "remarkable" or "incredible" and is a good choice when describing something impressive.
* 精彩 (jīng cǎi) - This means "splendid" or "brilliant" and is used for things like performances, events, or ideas.
The best way to choose the right word is to consider the situation and your relationship with the person you are speaking to.