I almost feel like I've done a disservice for not mentioning another fun and potentially infuriating -- for the other team -- hero. It's everyone's favorite hero, sometimes referred to as "Satan", Mei! I think I need to toss in another one of those caveats here, and that's that I think Mei is really only good on certain maps, but when she's good.... she's good. One of Mei's strongest abilities is to block of the enemy team, and if you're good at timing, you can actually trap some of the enemy team in front of your wall for easy pickin's.
Where do I like playing Mei? One of my favorites where I'll almost always switch is Nepal - Village. She's so good on this map, because her wall can block the main entrance to the central courtyard, and it's not that easy to go around. There are also only two large entrances to the point, and her wall can block them too. This is extremely good if you hear a dangerous ultimate go off (McCree, Soldier:76, Roadhog, Bastion, etc.) and you want to protect your team. Another good map is Hanamura. Although, the first point is definitely more effective due to there being only two entrances into the courtyard, and one is inaccessible to most heroes.
Now, she can be a bit hard to get used to because the way she deals damage is a bit different. Her left click is a close-range freezing mist that deals minor damage and freezes after 2(?) seconds of continuous exposure. Her right click is an icicle projectile, but what makes it hard is that it has a delayed fire. You'll see most people freeze a target and then perform a right-click headshot, which kills most non-tank heroes.