[The Might]

I say Treebeard

Above the trees and above the valleys
ִ̣̣̣̣̣̣̣ ִ ִ around the world I'm looking from high. - he sang songs about Beleriand, so he must have been around the world; and he is tall
Before the darkness I found my friends here,
ִ̣̣̣̣̣̣̣ ִ ִ being here before darkness was cast. - he was present from the beginning together with other old Ents
Calm touch of sun was still on my face,
ִ̣̣̣̣̣̣̣ ִ ִ clouds of shadow were not upon me. - everything was ok

Dangerous time has settled on the world,
ִ̣̣̣̣̣̣̣ ִ ִ doubt and doom coming to Wise men of old.
Even though High Man was in my sight
ִ̣̣̣̣̣̣̣ ִ ִ early he died, no one could help. - he might either mean the gards of Isengard killed by Dunlanders, or Numenoreans (as High Man is a translation for tarkil, which means Men of the West more or less)
Far from home he died, I am a witness
ִ̣̣̣̣̣̣̣ ִ ִ fear will fall on all because of this. - again, not sure who is meant by this

Grave times then came, and to me as well
ִ̣̣̣̣̣̣̣ ִ ִ great deeds were done, but I knew of naught.
Hope and its bringer I have seen from afar,
ִ̣̣̣̣̣̣̣ ִ ִ have been observing the tide that has turned. - not sure which bringer...Gandalf
I was lost yet though, abandoned by friends,
ִ̣̣̣̣̣̣̣ ִ ִ in creeping darkness I was left alone. - perhaps the Entwives that left

Kindness don't seek at me - from the dark times,
ִ̣̣̣̣̣̣̣ ִ ִ knowledge of me is knowledge of shadow.
My name was taken away from me long ago,
ִ̣̣̣̣̣̣̣ ִ ִ may someone yet still remember my name? - some do, but most don't
Now I am story of fear among once-friends,
ִ̣̣̣̣̣̣̣ ִ ִ name of mine recalls only tremor. - the Rohirrim feared the trees and considered them a legend

Over my garden I look still in fear,
ִ̣̣̣̣̣̣̣ ִ ִ oh, what if all of it comes into dark?
Prison of stone now stands where I be
ִ̣̣̣̣̣̣̣ ִ ִ past joy forgotten, cold stone is all. - Isengard and Saruman's machines
Really there's no way to lift curse of mine?
ִ̣̣̣̣̣̣̣ ִ ִ rare hope appears sometime - soon to be gone. - Gandalf that came to Fangorn and then again left possibly

Stars have been hidden, darkness will return, - as in the last riddle, Sauron's clouds
ִ̣̣̣̣̣̣̣ ִ ִ seven times stronger the darkness will come. - Sauron's return
There was the man who came to the wall,
ִ̣̣̣̣̣̣̣ ִ ִ to my gate came once weary wanderer. - Aragorn that came to Orthanc
Verily, was it just wisdom he sought? Or more? -
ִ̣̣̣̣̣̣̣ ִ ִ victory's key was what I gave to him. - the Keys of Orthanc, that were found during the Ents' victory

Warder I became to them who my Lord brought,
ִ̣̣̣̣̣̣̣ ִ ִ woe betide those who come to know me! - he made sure Saruman and Grima didn't escape
Years uncounted must have passed by me - quite old
ִ̣̣̣̣̣̣̣ ִ ִ "Yield!" I hear. "Darkness, leave him at once!"
Zenith of sunlight - can it be true?
ִ̣̣̣̣̣̣̣ ִ ִ zither and horn, I am free again! - could be interpreted as getting rid of Saruman

I know I haven't found matches for everything, but I think it's him.