Great poet?

At a recent poetry discussion, the subject came up of great poets. Who are they? W.B. Yeats was mentioned right away. Perhaps a more interesting question is that of great poems. A poet may not be considered great in the light of the body of work, but may have written several great poems. For example, Dylan Thomas may not be judged a major great poet, but his poems "Do Not Go Gentle into That Good Night"and "Refusal to Mourn the Death of a Child, by Fire, of a Child in London" are great poems.