An interesting question and one that I've never seen debated in the 100 Greatest ... type programmes. My guess is, that if there were such an undertaking, that Dante would just pip Ovid and Shakespeare to the honour but that's just my gut feeling.
The problem with this type of debate is that poetry, more than any other form of literature, is best heard in the poet's original tongue. So such a debate, if carried out in English, would be dominated by British, Irish and American poets, which seems unfair on the many excellent poets writing in other languages. The English-language poets that I would expect to feature, in no particular order, would be Shakespeare, MIlton, Eliot. Keats, Byron, Wordsworth, Chaucer, Whitman, Heaney, Hopkins, Yeats and my big favourite, Blake., but as 3styler says, it's all down to personal opinion.