Just so it's clear what Clay.io offers regarding advertisements: Right now we serve up a few different ad networks, trying to maximize the revenue depending on the device used (Leadbolt for example is one of the ad networks, and typically has much better results than, say, inmobi), as well as the country a player is located in (some ad networks do better in certain countries). Of course, we take a 10-30% cut (depending on where the game is being played) so there are definitely pros and cons :)
Eventually the plan is to roll out our own ad network so more money makes it to both the developer and Clay.io, but we need *much* more traffic before that becomes a possibility.
As Eisenhans mentioned, Leadbolt is mobile-web only, but from our experience, it's probably the best ad network for mobile HTML5 games right now.
If you can manage to not break AdSense's terms (something most games have problems with), they're likely going to give you the highest CPM.