For me it depends on the layouts and ad unit sizes. Text ads have always worked best for me, however in the last two days earnings and CTR have been s#it!
text links - can blend in content better, especially if you have long posts or articles. they can imitate a menu, a sidebar, or a reference at the footer. they indeed tend to pay out better and they don't stand out as an "advert" all that much image ads - perfect for sites like ecards, games, photos, etc. on such sites the CTR is very high (on my site : 10% and upwards) but the pay per click is poor, so in the end of the day, the high volume of clicks may not pay out.