Who was the Roman Philosopher who contributed the most, and What did he Contribute?

Answer 1

Let's try Marcus Tullius CICERO (pronounced "kikero").

He took on the massive task of introducing Greek philosophy into Rome.

Stoicism / Epictetus was Greek, not Roman.

Answer 2

Stoicism was the most influential of the Roman philosophies.

It's central tenet was to achieve the good life by denying the validity of human emotions (especially pride) and trust only to reason.

See Epictetus.