I think the formats doesn't determinate the CPC of an ad, I saw that image ads are paying low CPC but the rest adsense for content are the same..
The unit that pays the most is the unit that gets served the first ad, depending on how you have the ad blocks placed on your site.
In theory, the fewer the ads that you serve per page the higher the cost per click because the highest paying ads are served in the first block. Its a compromise between providing enough blocks to get higher click through and not serving two many that may dilute cost per click.
The adsense earning is totaly depends on the traffic. if u hav high traffic on ur site than u get paid more.
728x90 works the best for me, i fine that the 336x280 never works for me, i guess it depends on the type of website you have it on.
336x280 and 250x250 but most importantly BLENDING is the key here. A button can be highly effective if used properly instead of using 720x60 with lot's of white space.
A lot of webmasters = a lot of opinions. Basically it's all about testing to see what works best for your site.