General:
* Grand
* Monumental
* Heroic
* Legendary
* Extraordinary
* Magnificent
* Vast
* Impressive
Literary:
* Mythical
* Mythic
* Heroic
* Legendary
* Classical
* Timeless
Informal:
* Amazing
* Awesome
* Incredible
* Superb
More specific synonyms depending on the meaning:
* Vast: For scale and scope
* Heroic: For bravery and courage
* Mythical: For something of legendary or fantastical nature
* Monumental: For something of great significance
The best synonym for "epic" will depend on the specific context in which you are using it.