Quercus phellos, or Willow Oak, may very well be one of my top five trees. It has the classic character and fortitude of an oak, and its fine texture of narrow, willow-like leaves, gives it a modern look. Simply put, it's a gorgeous plant.
It also does fairly well as a street tree or in our city parks. Here's a shot of one in Tompkins Square Park:
Certain trees are particularly beautiful when you look up into their canopies. Willow Oak (as well as Sweetgum) qualifies as such. The leaves are willow-like, in that they are narrow and lanceolate (though not cerrated like Salix), but looking into the canopy of the tree always reminds me more of bamboo. I love the way the leaves sprout from the twigs in little bundles, like fireworks or unfurling hand-held fans.