1440p 144/165hz with G-sync is the best all around configuration. It can play every thing extremely well. Where as 1080p and 4k both are severely limited in some situations.
The low framrates of 4k makes most games feel sluggish at the cost of only looking slightly better than 1440p. And the 1440p monitors are so fast now that there is no real reason to pick up 1080p as long as your rig can run it. (unless you are an uber elite pro CSGO player which, if your wondering, you arent.)