Formal:
* Gr8
* Grt
Informal:
* Awesome
* Amazing
* Rad
* Epic
* Sweet
* Dope
* Coolio (older, but still around)
* Kewl (dated)
You can also use emojis like 👍 or 💯 to express "great."
Remember, the best way to say "great" in text speak depends on the context and your audience.