'Best' isn't a helpful word here.
4k clearly has the higher resolution, but it's also a radically more expensive technology and you won't be able to expect running games in 4k on high/ultra settings unless you have an absolute monster of a machine with at least twin high end graphics cards. Between that and the lack of content in 4k (at least in native 4k - for all kinds of reasons it's expensive and inconvenient to shoot video in 4k even at the professional level so even services that claim to provide you 4k mostly won't be able to deliver content that began at 4k and even then it's very very very demanding on your connection to watch it without buffering) there's no real reason to invest in 4k at the moment. About the only thing the average computer user could even display at 4k is digital photos.
It all comes down to cost and how much you care about having the latest thing to show off. It's not like a 4k display is strictly speaking a waste as such but for the time being it just seems like such a bad value proposition. For less than the cost of a 4k monitor you can get a 2k or 1440p 144hz monitor that rocks the blocks.