Find your balance
Do not ask the question, Which is the best position?
But instead ask the question
What do I want to get from the popup and how can my exit point help?
Most of the time a little bit of ambiguity will work wonders for advertisements. You want to challenge the typical location of things, this is to escape falling victim to sleep-walking behaviours (or more simply, people who instinctively click away before looking). You have to balance difference with annoyance, you don't want it to be too easy to navigate around, but you don't want it to be too hard as well.
Apple does this well, and they know it. You are used to seeing the exit button in the top right and so when this popup displays, it forces you to 'search' for the button, which may only be a few seconds to the average user, but is a large deal more productive/aggressive than the 'instant closure' of a user in autopilot.
Note: Apple users will instinctively look to the top-left, but I would say there is a good chance that Apple uses this correlation in lieu of challenging Windows users, after all, it's for a piece of hardware that is staple in converting regular PC users to Mac products.