Easily Witcher 3 Wild Hunt, loved it so much I even NG+ed it, and redownloaded it on several computers (after being forced to do so for various reasons)
Beyond Witcher, it would have to be GTA V, but I'm sure maybe a quarter to half of the time I've played that game is spent in what my friends like to call either Sky Simulator or Black Screen Simulator, ie loading screens nonstop.