Now this is a toughie. My knee jerk response would be John Dunn. There are so many great poets past and present that to single one out is unjust. I can say that my favorite Poem is by W.B. Yates:
An Irish Airman Foresees His Death
I know that I shall meet my fate
Somewhere among the clouds above;
Those that I fight I do not hate,
Those that I guard I do not love;
My country is Kiltartan Cross,
My countrymen Kiltartan's poor,
No likely end could bring them loss
Or leave them happier than before.
Nor law, nor duty bade me fight,
Nor public men, nor cheering crowds,
A lonely impulse of delight
Drove to this tumult in the clouds;
I balanced all, brought all to mind,
The years to come seemed waste of breath,
A waste of breath the years behind
In balance with this life, this death.
This poem ran through my mind endlessly while I was in Iraq in 03/04 and I often think that it brought me home. Having no fear of death, or rather finding peace of mind no matter the lack of justification for what I was doing was very important. I came home and became active to vote out the criminals, although unsuccessful, at least my voice was heard by some.
On hubpages we have a vast array of talented poets all of whom deserve mention, but I will not venture the attempt here. I urge any reader of this to seek them out. They are there, and their words will lift your soul, break your heart, or fill you with rage. The passion of poetry is alive and well here.