Its hard to pick one favorite tree. Most of them have already been mentioned. I was mesmerized looking at a stand of 40 ft. tall birch trees in the yard last autumn. The leafs would just shimmer in the sunlight when the wind rustled them, so relaxing. Hard to find a picture of that though.
I'm surprised nobody has mentioned giant redwood trees, so I will.
Massive
General Sherman [possibly the most famous tree in the US]
peace.
seedless

I'm surprised nobody has mentioned giant redwood trees, so I will.

Massive


General Sherman [possibly the most famous tree in the US]

peace.
seedless