The odyssey, Beowulf, Gilgamesh, Homer's lliad
Epics are divided into two categories: primary, which are initially oral and only later written down, and secondary or literary, which are composed and written down at the start.
The most famous primary epics are Homer's Iliad and Odyssey. Although these are the major remaining Greek epics, there were many other epics sung at the time which are now lost, and are known only by references to them in other works. It is believed, for example, that there were epics covering the entire ten year period of the Trojan war, but of these only the Iliad, covering a relatively short period during the last year of the war, has survived in more than fragmentary form.
Other primary epics you may have read or at least have probably heard of are Gilgamesh and Beowulf.
Primary epics were memorized and recited by professional bards. For those of us who find memorizing even a fairly short poem or passage a challenge, the ability to memorize thousands of lines of poetry is an impressive feat. The Iliad, for example, is some 12,000 lines of poetry, and took two full days (about sixteen hours, not including meal and rest intervals) to recite in its entirety. By contrast, Hamlet, one of the (if not the) longest roles in all of Shakespeare, has only about 1,500 lines. And the Iliad would only be a part of the repertoire of a Greek bard.
Some of the best known secondary epics are Virgil's Aeneid, Dante's Divine Comedy, the Song of Roland, Spenser's Fairy Queen, and of course Milton's Paradise Lost. Some authors include Ovid's Metamorphoses within the category of epic, but I find that it lacks the coherence and flow of narrative that I look for in true epic. But that's a matter of opinion.
Other epics of India, Persia, and other cultures are less familiar to most Western readers, and are unlikely to have had much influence on Milton.