Willows are lovely. I love how they sway, I love how they are the emo-bangs-haircut of trees, how you can weave their branches into baskets. Their smooth bark and silvery leaves, how their presence signals that water is close by. They are fey trees; if dryads exist then I am certain they are kin with willow trees.

Oaks are a close second. Big, tough, slow-growing, gives acorns back to the forest denizens, leaves are fill out character. They are the old grandpas of the forest, nurturing the younger life forms, offering them nooks and crannies to nest in, and eventually donating their bodies to the bacteria and fungus in the soil. They also make the best structures for tree houses, in my experience.