We can admit it, we all have our favorites in just about everything, and though the definition of favorite implies that there is only one, I feel you can have multiple favorites, one from each of various styles and makes. So while this definitely plays a role, and is rather obvious, we still need to consider the other points.
Size and Shape:
I never realized how much size and shape really play as a role in how I pick out my teaware. Today I was having a session using a Gigantic cup, that left me borderline frustrated. While the cup does hold a large amount of liquid, the fact that it holds that much causes a very reduced cooling time, something which must be taken into consideration if you are cooling the water for the tea in the same cup you will be drinking from. But not only that the cup was so big I was having my normally rather large steeps, and it left the cup looking almost vacant. The cup was just Huge for the purpose for which I wanted to use it.
But while size is important with cooling, so is shape, I have a rather favorite Yunomi by Mukuhara Kashun which during cool weather I felt my self grabbing nearly every time I wanted to have Sencha. But now that it is warm because it is a Classic Yunomi shape instead of the Wan (bowl) type shape that I have much more of, it cools a lot slower. It lends credence to the idea of different shaped Chawans, and the standard summer Chawan appearance of incredibly wide at the top slopping in slowly to the center and bottom of the bowl.