When it comes to screen resolution Full HD screens are dominating the market as of now, but the trend is vigorously shifting towards Quad HD i.e. 2K screens. Yes, companies have started to shift to 4K screens but very slowly. In fact, there is only one phone, the Sony Xperia Z Premium, that has a 4K display. Since the trend is shifting to 2K displays, hence the list of all the 2K phones out there.
Quad HD i.e. 2K has a resolution of 1440×2560 pixels. It would only be effective if you have 2K media to watch else it would just be eating up your battery. But it still good to know that you are capable of viewing 2K media.
Google Nexus 6