WINDOWS: If you have a powerful desktop, then Adobe Premiere Pro CC would be my number one choice. The only downside to Adobe Premiere is that it requires a lot of resources. If you are looking to edit video on your laptop, then my choice would be Cyberlink PowerDirector. PowerDirector is simple to use and runs great on my laptop (i7, 8gb RAM, integrated graphics). It's great for quick editing in your car.
MAC: I don't use a mac, but I know a lot of Mac guys love Final Cut Pro.
LINUX: Lightworks is my favorite for Linux. Like Adobe Premiere, it's feature packed (which also means it's complicated). It runs faster than Adobe Premiere though (probably because it's running on Linux), and while it's still in Beta, it's also free.