Number 1 would be Xbox One, but a strong second being the Steam Controller
Xbox in terms of sheer quality, comfort and universal support in PC games all alike
Steam Controller in second due to it's iffy quality but it's sheer adaptability and control customization is amazing, if you're fine with launching games in steam big picture mode
You don't have to launch them in big picture anymore. You just have to have the controller turned on before launching.
But, while the controller is very adaptable, it's not easy to use. The 'd-pad' is not a very fun d-pad, and I find using it very uncomfortable for sidescrollers or any game that requires it regularly. The right pad works well for a select few games, games like Dynasty Warriors and such. It does not feel good to use as a stick, and the 'mouse brown townog' mode only really works for games that use the right stick as a camera control. In practically any game except melee beat up games where 360 degrees of movement is better than 8, you're better off using keyboard and mouse.
Steam also now allows you to customize xbox and ps4 controllers as well. Giving you gyro and touchpad control with a PS4 controller.
Poll