That's a loaded question.
AdMob is the easiest to set up to show ads in your app, assuming you don't care what ads show. For direct deals, you can set up house ad campaigns in AdMob and track impressions and clicks and charge the advertiser that way, but it's more of a one-off solution. Small (especially 1 person) shops generally start out with AdMob.
Doubleclick for Publishers (DFP) has first-class support for direct deals, and can do forecasting and more complex targeting. It can also backfill to AdMob or Ad Exchange for any unfilled impressions. Generally, DFP companies have a dedicated sales team that goes out to get deals to place into DFP.
Unfortunately I don't know as much as I should about Ad Exchange. But if you aren't already using it, it's probably not the place to start. There is also more overhead involved than signing up for AdMob.