Most formal languages are created with an specific purpose, so there isn't simply a "best" one. You normally use tons of them in academia, and many times simultaneously.
If you give more information about what you are looking for, we may be able to help you.
If you just want to learn something interesting though, I'd say typed lambda calculus is a great option.
If you give more information about what you are looking for, we may be able to help you.
If you just want to learn something interesting though, I'd say typed lambda calculus is a great option.