A WebmasterWorld thread has AdSense publishers complaining about the worse ads they've seen on their own sites.
One of the top and most annoying ads on the list is the fat belly ad. Others also say they hate the diet-ads with the banana in the ad. Some others hate the background check and auto loan type of ads. While some dislike the retargetted ads because they've already purchased at the site and no longer need to see ads for a product they purchased.
How do AdSense publishers block them? Well, it isn't easy. Netmeg said:
I've found that for display ads, the most effective way for me is to save a copy of it, upload it to the ad center, apply the filter to find others like it, block all those, AND click on the link that says "find related ads" or whatever and block all those too.
But invariably they come back with a new URL a few weeks later.
Forum discussion at WebmasterWorld.