To me, an interesting poet is someone whose poetry creates effect. For example, when I first heard Sean Bonney recite packet switching poetry (to invent the wrong name), the jerky rhythm & the subject chop made me feel strongly of London, the nature of place, my experiences there.
A very interesting poet is a poet who finds a new way to create effect. I don’t know to whom I can ascribe packet switching poetry, a number of poets used it even then. It seems to be coming quite popular now. But whoever came up with it will be, to me, a very interesting poet.
A great poet is someone who creates an effect new to poetry. That’s why I put Prynne in the great category, he’s found a way to create something in poetry I’ve only otherwise felt in music.
The “mainstream” doesn’t seem interested in exploring new ways to create effect, so, by my argument, a mainstream poet can never be more than interesting.
Equally, though, by this argument, I have to allow a poet who’s doing something I don’t expect, or don’t like, a chance. Almost by definition, the people who find something new to me are those who’ll be doing things I don’t expect. Thus I explore the mainstream, the buggers could surprise me! Much more, though, I seek out the new and the different, especially translations. I try to avoid dismissing something as drivel automatically (although if it walks like a skunk, & it quacks like a skunk, …); in among the 99.99% awful may be something incredible.