Tubebuddy is a great way to see what tags are suited.
Besides that check your analystics and see wich words are used to find your video's. Besides that use everythibg that is relevant to the video. For example if you do Call of Duty Black ops 3. Cod Bo 3, Cod Black Ops 3, Black ops 3, Black Ops, Black, Ops, Call of Duty, Call, Duty.
I learned that using that by seperating the words it can be found easier.