The trick with choosing the Ad Format that you are going to use is to actually choose it and not let your current website structure which you built before you decided to monetize your website with Adsense choose it for you.

This is what the average Adsense Publisher says to him or herself - "This is how my site is built - I can make that space available there and so I'll have to place Adsense there which means I'll have to use that ad format". This is not a choice and needless to say it will generally not convert aswell as rejigging your page layout so as to accommodate the ads that will perform best into it. What this also means is that placement is more important than format.

The basic Ad Format rule is that wider is better and Google gives a neat explanation of why this is so here. Note, wider doesn't mean width of the whole ad (otherwise we'd all be using leaderboards) but the width of an individual advertisement inside the ad block.

This means that the large rectangle - 336 x 280, the medium rectangle - 300 x 250 and the wide skyscraper 160 x 600, other things being equal all perform better.

In content sites I always try to use at least one large rectangle but remember that a webpage can often accommodate more than one ad unit.