Adsense is probably the best, they have more advertisers than anyone else I would think. YPN is probably second.
Then there are a ton of the small guys out there. Sites like Bidvertiser, SearchAnyway, Textervise, Kosanni, and you can get lucky with some of them too. Some have great bids, but maybe not the selection of ads that Adsense will have.
I usually use at least 2 or 3 different ad providers on all my sites. I hate tossing all my eggs into one basket.