16:9 of course. 4:3 is for the dinosaurs. 99% of TV's in the stores are 16:9. Why we need to support old standard? All projectors, LCD, plasma, etc. are 16:9. And of course it must be in high definition (1280x720 should be enough).
Kurdt, if u for some reason has to run MP on your computer screen, I would guess 4:3 shoult be the right size... but of course, 16:9 is most used for TV...
When such a nice skin is developed, it should be both 4:3 and 16:9...