Pretty much anything that says download or the belly fat ones. I've seen the background check and auto loan ones elsewhere and disliked them.
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.
Ditto to the above mentioned offenders though I prefer to run with text ads almost exclusively these days. On the text ad side of things, any ad that dominates my pages regardless of the page content to the point of shutting down my revenue stream qualifies. When clicks dry up I go looking for these and, when identified and banned, things usually loosen up a bit.
As above. "Download" and "flat belly" are the scum of the low quality ads.
Ads for products I've already purchased on eBay or elsewhere, weeks earlier. Totally pointless.
As a matter of interest, I've never clicked on an AdSense ad nor have I ever purchased from an advertiser's site.
As much as we might hate them, they are effective. Diet products have a massive pull and, at least my demographic, clicks on them like crazy.
It is the wrong time of year, but when the political season gets into full swing (and gawd help me when it is a presidential election year), I HATE political ads. They are full of vitriol half the time and I also end up with scathing emails about "how I am supporting candidate X" when I have no control over it. So political ads get nixed pretty quick.
Apparently I need to surf the web more. I only recall seeing any fat belly ads once and that wasn't on my site.
Years ago ad targeting was a lot worse, for me anyhow.
Speaking of political ads, some ads are not overtly political but the companies behind them represent a position on a polarizing topic or lifestyle choice.
I have received numerous complaints about a specific corporate advertiser that many site visitors object to. While I don't agree with the viewpoint/lifestyle represented by that corporation, I have not blacklisted them because it might depress the auction.
The political stuff has occasionally been an issue, and I've been known to ban an ad if it I thought it would be particular offensive (to either side).
For my direct advertising, I have specified that I will accept candidate ads but not issue ads, as my sites are not the least bit political in nature and I don't need the agita. Probably will do approximately the same with AdSense, next time around.