As far as I can tell, the only 100% accurate tracking is occurring with Facebook Mobile App Install ads since they have an integrated solution with the App store. But there are many 3rd parties that claim to do it, but my understanding is that it's a "correlative" confirmation, not a true confirmation. Which is the best?
Sure, it's correlative for technical reasons, iTunes / AppStore does not provide (yet, might change) a mechanism for that, so 3rd parties have to build their own.
I recently discussed with people from Adjust and I was really impressed.
HasOffers / MobileAppTracking seems to be a good choice as well.