Buying a controller by the end of this year. Which one to buy?

I personally use a DS3 with the scp server application. It works for every controller compatible game for me, and I'm pretty sure the DS3 is pretty cheap at this point. The main reason I use it is because I barely touch my PS3, and I have a second controller anyway. Plus with this you don't have to go with that one shifty driver installer.
Edit: One other plus is that you can use a Bluetooth dongle or something to have it be wireless.

Xbox 360

Got a wired 360 controller last Christmas and loving it so far. Allowed me to relieve my couch-gaming days when I was a kid. In terms of compatibility, every controller-compatible game I've thrown at it has worked flawlessly. Design-wise, the transition from a PS2/PS3 controller may be awkward for some people given the location of the D-pad and left analog stick but setting that aside, it's a durable and awesome controller.

It's perfect for games action/rpg games similar to Castlevania: Lords of Shadow, Witcher 3, or the Dark Souls trilogy, while it may take some time in getting used when playing platformers/shooters such as Dead Island, Nuclear Throne, Enter the Gungeon, and lastly playing fighting games isn't so bad. I play Tekken 6 on an emulator using the controller and the D-pad is as someone stated above, accurate but tiresome.

I do personally use a Razer Sabertooth Elite controller and, as I know, it's one of the best. The little thing that makes a difference is the removable "usb-cable" cause we know it can be a real problem on other controler. But it s just an opinion of course! GL!

Xbox 360 controller
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Logitech to avoid all those Nonsense QQuing Xbox Controller Complaint Threads being spam on the forums.....

I use a 360 controller. Your best bet is that because it's the most supported controller, so you would also be fine with the Xbox one controller too since it's not like the buttons changed. The 360 controller is much cheaper but if you can afford it, go for the Xbox one controller. Wired or wireless doesn't affect anything either. The drivers are also already installed so it's plug and play basically. Good luck, gaming with a controller can be really great :)

I use PS3 controllers because I bought a PS3 several years ago. There a slight hassle thought because you have to use motion joy to get them to work. If I wanted to drop $75 or whatever I'd probably try out the steam controller or I'd get an xbox controller to remove the hassle.

I personally use the Steam Controller, mainly because I like the ability to customize the controls to fit how I want to play the game. If, however, the customization aspects seem like more of a headache than a feature, maybe go for the Xbone controller.

Everybody will recommend you Xbox controller for some reason, but for me it's worst controller ever, even NES controller is better than that. I personaly use PS3 one and it's really great but as said above, you'll have to use motion joy software.
(You have to use it once, everytime when you plug it in so if you let it plugged in you don't have to touch it.)

If you want a wireless controller Microsoft released a new wireless Xbox One controler a week ago and it has bluetooth so you won't need any adapter like with the xbox 360 one. If I'm wrong please correct me