Probably my all 'round favorite is Poplar. It is consistent, and I can get it most anywhere (i.e. HD and Lowes). The aforementioned stores even have it in convenient selections of 1/4", which is the vast majority of what I use in projects. The color can be a bit inconsistant at times, but when you get a pure piece, I think it is very beautiful. I am even tunrning poplar pens now and love the way they look.
For thicker stuff, I really enjoy working with walnut, ash, and especially cherry.