My first favorite tree was an American Chestnut that I used to walk past on the way home from elementary school. For a time there was a slate sidewalk underneath the tree and the chestnuts themselves were a constant annoyance, but the tree was big, beautiful and shady, it made my neighbor's yard look really nice. I was shocked to learn later how fortunate I was to experience a real American Chestnut that had survived the blight (reddit even has a sub dedicated to finding survivors: https://www.reddit.com/r/americanchestnut/). I went back to try to find pictures of the tree and its since been razed. Presumably it died of blight, and the owners replaced it.
My second favorite tree is a somewhat famous Giant Sequoia I found after moving out west. I'm not doing disclose the location or the tree's name because its too easy to figure out where I am. The tree is the largest in town but not by much... it does stand out though. Without knowing its species the tree got me curious. I found out that it was actually a Sequoia! this surprised me because I thought they were relegated to small areas inside of national parks - NOT TRUE! Finding this tree led me to be intensely curious about all the other trees in town- what else was I missing? so I spent 6 months going on morning walks with a tree identification book. I met all sorts of people, and learned a ton about the trees. Tree identification is super hard by the way... I'm still getting my feet underneath me even after all that time. I was able to identify ~30 native species though, which was really cool!
I'd love to hear about other people's favorite trees.