7. League of Legends
There are a few things we need to reconcile with gaming: if you're playing with anyone online, odds are they're going to be an awful asshole at some point. When you're not sitting in the room with someone, they're simply allowed to be more of a dick, because there's no actual consequences. So when you mix online play with teamwork (where, ya know, everyone needs to get along and work together), things can get hairy very quickly. League of Legends has this problem ten times over - and has become something of a legend for the number of hurt feelings its caused. Hell, there are actually Youtube videos of people fighting over LoL and using internet words during the argument. Honestly, saying "butthurt" in real life would be enough for me to end a friendship. You know what I'm talking about, Lauren.
6. Worms (series)
Worms is sorta our modern equivalent of chess: a slow turn-based game, all about putting the right pieces in the right places, and you have to be very careful with your ninja rope inventory or else...wait, nevermind. But while it's like chess in a lot of ways, it's also slightly different (shocking, I know) - by having more than 2 teams playing at once, it's very easy for one player to feel like they're being ganged up on unfairly (even though your guys were clearly super close to the water, Bobby). And just like chess, a mad sprint towards victory will make you shed any form of kindness in order to attain your goal of not having your virtual worms get drowned in okay this metaphor isn't working at all. The point is: that was a lucky grenade throw, Bobby, and you need to apologize for your bullshit.
5. New Super Mario Bros. (series)
Co-op Super Mario Bros. used to be a real chore back in the day - only two people could play, and they had to take turns with each death, never really getting to do anything simultaneously. New Super Mario Bros. changed all that, allowing for 4 player co-op throughout the entire game. And with it, 4 players yelling at each other, complaining about stealing items, getting 'accidentally' tossed off a ledge, etc. Sure, we may be "bros" up on the screen, Tyler, but in the real world, you're just another Koopa as far as I'm concerned.