Due to variety of tree sizes, shapes and attributes it is impossible to determine the best tree for your particular need in a website.
In preparation for a conversation with your Aborist or Nursery Person you should consider the mature size and maintenance requirements for the tree. Consider the pros and cons of shade vs sun. The effect of the tree on views, privacy, utilities, and the surrounding garden. Do you want an evergreen or deciduous tree? Do you want fruit, flowers or fall colors?
A good starting point is the Sunset Western Garden book. I recommend a walk through an arboretum with a notebook for names and characteristics you enjoy, and some time spent researching the trees you find attractive. With that information in hand you are ready for a consultation with an Arborist or landscape professional.
"only an optimist plants a tree" Peter Brooks