Overwatch's Roadhog might be listed as a tank, but he's best used as a tank killer. His self-heal makes him independent and easily able to handle fights outnumbered, and his Chain Hook ability will have everyone rethinking how close to you they should get. Against all other tanks, Roadhog's high-burst damage is a big threat.
Scrap Gun [LMB/RMB] - Roadhog's Scrap Gun is as painful as it sounds. With the LMB he fires a shotgun blast of metal pieces that will devastate weaker heroes in one shot and with the RMB he fires a less deadly medium ranged blast. Roadhog is slow, and as you'll see, just about everyone is going to want to avoid you like the plague. Get used to using RMB to fire the scrap gun until those rare moments you catch someone out of position and can unload a junkyard into their face.
Take a Breather [E] - Roadhog's self-heal, Take a Breather, is what really makes him so deadly. Combined with his staggeringly large health pool, it takes a coordinated effort to put Roadhog down for good. Like Mei's self-heal, Take a Breather is a channeled ability that heals Roadhog rapidly but not instantly. Unlike Mei, he's not invulnerable during the process, so using it in the heat of battle might not save you. It's best to use it during a lull in the fight before heading back in.
Chain Hook [LSHIFT] - What really makes Roadhog such a feared hero is his ability to hook heroes and pull them close in for a deadly finisher. Chain Hook will require great aim and an understanding of how long the range is (my experience tells me it's always longer than I think it is), but once you're familiar with it people will begin to hate you. With Chain Hook, pulling foolish enemies out of position is incredibly easy and you can use it to clip Pharah's wings or even interrupt certain ultimate abilities. When the enemy is pulled in, follow up with a Scrap Gun blast to the face and a melee hit (default key is V) to kill most heroes outright.
Whole Hog [Q] - Roadhog's ultimate ability is best used to counter enemy pushes rather than earn kills. When it's triggered, Roadhog begins firing his Scrap Gun like a chaingun where each hit pushes enemies back and does moderate damage. If you find yourself defending against a heavy push to win the match, Whole Hog can give your team the space they need to regroup and secure the win. It can also be used on certain maps with lots of drops to push enemies to their deaths.
Roadhog Strategy Tips
Roadhog's whole purpose in life is to pull heroes out of formation and murder them, but accomplishing that is going to take some steady aim and patience. By now, everyone is aware of what a threat Roadhog can be up close and will be looking to avoid you at all costs, use this aversion to your advantage and always put Roadhog on the objective to force fights from other heroes.
Chain Hook is what every enemy is afraid of but it's only useful against heroes that don't see it coming or those that lack certain dodge abilities like McCree's Combat Roll. When selecting targets to hook, it's important to consider that Chain Hook can also interrupt channeled abilities like Pharah's ultimate, Barrage.
The other thing about Chain Hook is that you need to be aware of where your team is when using it. Nothing will make people hate you more than hooking a self-destructing D.Va or accidentally pulling a Reinhardt right into your offensive heroes. This is why Roadhog players need to understand whether to squat on the objective or pull away to pick heroes off one on one. Either way, Roadhog is brazen and rude, so putting yourself front and center is always a good place to start.