Everyone said 2016 like ps4 was going to go into terminal decline. 2017 looks to be bigger. 2018 has the best games line up though and maybe Sony will have bigger price cuts in 2018. Even if they cut the Pro by $100 and the slim by $50 this November the benefit of those will still be seen in 2018 as it gets a full year at the cheaper price point. And who knows maybe another cut in 2018.
So while I feel 2017 can beat 2016 it's still possible that 2018 beats them all. But it doesn't really matter which is the peak because they're all massive years. It's hard to see 2019 beating 2016/17/18 but if gta 6 arrives then who knows. Regardless of which year is the peak the years after won't be seeing massive declines until the ps5 releases which imo won't be any earlier then 2020. This machine is a sales beast and not a fad.
There's debate between 2017 and 2018 being the peak year but i think we can safely assume that there's 0% of chance 2019 being the peak year.