The *best* content you're going to do is the games you like. If YouTube didn't exist, what would you be playing? If you're not passionate and knowledgeable about something, it's going to show in your videos. If you target a popular game, you will be lost in thousands of people that are trying to cover that. My channel is mostly devoted to one game (Warframe) and while it's not among the most popular games, it does have a pretty strong following. Believe it or not, it's actually easier to be successful in a less popular game, if only because the competition is smaller. Warframe was harder for me because it does have a strong following, and the main YouTubers have been established for quite some time. But I've put in about 3500 hours in it and kind of found my niche. Because of this, there is less pressure and more time to develop what I should be doing. I also developed a plan and started executing it. This approach has been successful to where 2 weeks ago I had 320 subs, and am currently at 2250.
In short, play what you want to play because you will enjoy it more, and you will be able to develop much better.