Wow! What a question! I think it will take me all night to answer this.
However let me attempt something here.
For me the best poems are those which come from
(a) some authentic inner place of understanding - things have been resolved - and the poet is offering wisdom. You know those kind of poems - full of philosophical insights and memorable quotes.
(b) some incredible inner pain, that usually can be shared by the reader and has a truism about it that shines through. The language and imagery would, probably, put some shape and tone on the poem.
(c)the poem has incredible rhythm and flow to the use of words - again it would, probably, be about the choice of words and length of lines that match the 'music' of the piece.
and finally (d) - 'cos I'm running out of ideas - a very controlled editing process (the shape and form and use of language etc.is faultless) and yet the wholeness of the poem does not have a contrived aura, but flows beautifully as if straight from the pen, unedited - the work of a master. You just know you're in the presence of one such elite poet.
That's my contribution - hope you can follow. Never really analysed the process before.
There's probably a great academic answer to this question, already uploaded and I don't recognise it.