This is a very tricky question, because I love different pieces of teaware for different reasons. I clicked Japanese on the poll because my favorite tea cups seem at least Japanese-inspired: relatively subtle rhythms in glaze and form make that make the pieces interesting, 'natural' features reminding me of rocks, trees, water, reflected and leaf-filtered light. And some of my favorite pieces are in a japanese-inspired style but by American or European potters.
But my most-used brewing vessels are my simple chinese porcelain gaiwans, although I find my small glass chinese teapot and my tokoname kyusus to be more attractive overall, and handier for certain teas.
And though for practical purposes I make large quantities of tea in a plastic Kamjove kungfu art cup, and drink it from my thermos cup, these are matters of convenience and not preference [written while drinking some Haiwan Purple Bud sheng puerh from Norbu, brewed in a Kamjove, carried in my thermos to a meeting, and drunk from the thermos cap].