I liked the exchange that grinch posted, though I think that the attempt to connect it with philosophy doesn't hold much water. Not to say that philosophy isn't interesting, simply to say that poets are more interested on the whole, by other poets than by philosophers, at least when it comes time to put pen and paper together.
My notion is much the same as the unnamed poet who write the short essay that grinch quotes, that is, that you can take a shot at a sestina, a sonnet or a free verse piece on any given day, depending on what the particular piece seems to need. Forcing yourself into a box before having some sense of the poem has always seemed to be a way of shutting off certain poetic directions. That may be a help for you in your writing. For me, I need every break I can get.
I also like the list of more formal folk that Grinch's author offers. Richard Wilbur is another of my American Favorites. Not too many people talk about John Berryman these days, though he is spectacular if somewhat difficult at times. One of his early Dream Songs begins
"Life, Friends, is boring." He is a lesson in advanced metrics, and is one of the most amazing religious poets I've ever read, especially his 11 Addresses to The Lord. I've always found religious poetry difficult, for the most part, and often maudlin in the extreme, but this series in particular makes my hair stand up.
Some of his earlier poems are more standard in their metrics and rhyme, and they're to be found in his Collected Poems(1937-1971). His metrics are up to the standard of Milton and Shakespeare's in that early stuff, and as he gets older, he experiments with variations, but he's always clear exactly what he's doing. He's got a series of sonnets that I suspect he meant to stand against the Shakespearian sonnets, and their metric is fully as compacted and jammed as the 16th century models.
I'm taking up other people's space here. I'll shut up now.