2016 is when PS4 really picks up, but 2017 is when it'll shine at its peak.

I think it will be 2016.

2016 sales will be better than 2015, but the peak year will be 2017 imo.

I think 2016 will be its peak. It was stuck at 400$ for a majority of 2015, even though it got the holidays at a lower price. Next year it will be 350$ for the entire year (unless they drop it further), and it will get a lot of exclusives. In addition, next Cod will (most likely) not have last-gen versions, which will help the PS4 particularly much because of the marketing deal.

yes but if gta 6 is gonna be in 2017 , and the ps4 will be a 249$ or less , then its peak year will be 2017

Hard to say. My guess is 2016. Those suggesting 2017, although a pssibility, are forgetting to factor in the competition. The NX is the wild card in the equation. It can have a negative effect on ps4 sales.

2016 or above

2016 or 2017.
2017 if home consoles are resurrected by the ps4 in Japan, following a good push from japanese publishers in 2016.

Considering this upward trend, I don't really see it peaking this year.

I think next year will be peak, with the following years showing slow declines, so nothing dramatic like the Wii declines. Next year we'll see releases and/or announcements of some of Sony biggest hitters. If FF15 is good, that'll help greatly, especially in Japan. And we'll see an official drop to $299, with a possible Slim model.