Steam, GoG, Humble Bundle, and Itch are the platforms I regularly buy PC games on. By far and away, Steam is #1. It's got the games, it's well integrated into everything, easy to use (especially on Linux). GoG is good, although I set up Heroic to use with GoG. It's close, but not as many games that I want, and Heroic+GoG is a little less polished than Steam. Humble Bundle and Itch are generally too niche, although so much of my Steam library has come from Humble.