The build in plugins work fine. It depends a little on what kind of game you have and how aggressive you want to push out ads. Just regular banner ads work fine, rewarded ads can be really good if you have something to offer the player as a reward. Make sure it´s something they want over and over again (like extra money) One thing you often see is a fullscreen ad on game over (not every game over, but probably every 2-3 game overs) fullscreen ads are a bit too aggressive for my personal taste, so I went with a simple banner ad.
But rule of thumb, you mainly get paid for impressions (aka views) so make sure that ads show up often enough but not too often and not too aggressive.
I´d say just take the plunge, try stuff out and make your own experiences. The returns are very minimal unless you have a huge playerbase. Expect returns of a couple of cents. That´s at least what my experiences are so far. Oh and if you have a really good game with good ads going, I can also say putting out ads with google works really well.