I have very recently got into Rocket League, after it was released for the Xbox One, which is perhaps partly why it has made this list (it's still fresh in my memory). I was immediately impressed with the friendliness of the player base and how the idea of fair play is not yet dead. The whole point of the game is co-operation and team work with an entire mode made separate for those who wish to go it alone. Players very often congratulate each other for good plays and goals, even when it's the opposing team who has scored. It's always nice to see 3 or 4 'Nice shot!' or 'What a save!' messages pop up when you score a goal or prevent your team from losing.
I was also involved in one particularly chivalrous moment. I had just started a 1v1 game when my internet decided to have a nervous breakdown and I suddenly started lagging all over the place. My opponent easily scored his first goal before asking in the chat if I had lag as my car randomly slid around the arena. I said yes and he said he wouldn't play until my ping was back to normal. Moments later when it sorted itself out, he allowed me to score a free goal so we were back to level scores and continued the match.
Even though I went on to lose the game, this kind act resonated with me as a prime example of how gamers should behave toward each other.