Roadhog is a hero that does massive damage at close quarters, so staying away from him and his hook is a must, if you don't want to get killed. Moving around is best, as his weapon shoots significantly slower than most weapons. Calling out his position as well as his Chain Hook's availability are good tactics to use.
Ana can completely negate Take a Breather for a while, put Roadhog out of the battle, and have other teammates deal with him, which makes her a great counter to Roadhog. Her unusual hitbox also means she can usually survive being hooked.
D.Va's Defense Matrix is a major problem for Roadhog, as it can prevent him from killing targets reeled in by his Chain Hook, and is also able to negate up to 4 seconds of Whole Hog, meaning he is very vulnerable is he decided to use while she still has her mech. While D.Va can be hooked to temporarily stop Defense Matrix, she is still a problem to him if Roadhog's teammates don't focus fire towards her.
Genji has a lot of mobility, and is also able to out-range Roadhog. Due to Roadhog's short range and slow fire rate, both of these will be troublesome against Genji. He can also use Deflect to dodge Chain Hook to turn Whole Hog against Roadhog.
Pharah's Hover Jets makes it hard for Roadhog to do damage to her effectively, and her movements make it hard for him to hit her with his Chain Hook. With the range advantage as well as lack of falloff on her Rocket Launcher, Pharah can deal large amounts of damage to Roadhog from a distance.
Sombra's Hack will make Roadhog's job much harder for him and while she can't 1v1 him very well, she makes him easy pickings for her teammates. Additionally, hacking Roadhog while he is using Whole Hog will interrupt it.
Roadhog's HP is too high for Mei to take him down effectively, and can Take A Breather to negate most of the damage he receives while frozen. If Mei gets too close, Roadhog can blast her with his Scrap Gun, and if enough pellets hit, will be able to kill her. If his Chain Hook is available and hits, Mei will be either dead, or weakened to the point where Cryo-Freeze won't heal all the damage.
Roadhog usually looks for targets to pull with his Chain Hook, and Mercy is one of his priority targets. If she's caught in the open and is hooked, she very likely won't survive. Because of his high health pool, Mercy can't fight back effectively, either.
Lucio also has rather weaker damage, which means he cannot do much to live. However, Lucio can knock Roadhog away, saving himself from fatal damage, but is vulnerable to Chain Hook, which, if it hits, temporarily get rid of Lúcio's mobility.
Roadhog can go tremendous damage to Reinhardt's Barrier Field with one magazine, and will force him to back off in the scenario. Roadhog's Take A Breather also negates all damage done by a single Charge, and is also able to interrupt said ability with Chain Hook. Reinhardt's reliance on getting close to his enemies means the knockback from Whole Hog will be very effective against him, and will also tear down his barrier in 2-3 seconds if it as full HP.
Roadhog can kill Winston in a similar fashion to Reaper (another one of his counters), albeit with a slower-firing gun. Roadhog can use his Chain Hook to bring Winston in for massive damage, such tactic also preventing his escape. Roadhog's Whole Hog can keep an enraged Winston trapped in a corner, and does more than enough damage to kill him while still having 1 or 2 seconds to spare.
Bastion can easily shred Roadhog in Configuration: Sentry, and is particularly harder to deal with, as Bastion has 100 extra armor, self healing, and the Ironclad, which makes it difficult to take down while in Configuration: Sentry. However, Chain Hook is an excellent way for Roadhog to pull Bastion towrds him and pull it out if out of its Sentry form. Two shots will usually finish it off if the hook hits, regardless of what Bastion does.
Orisa can weaken many of Roadhog's attacks with Fortify, and she has a much higher rate of fire, which means if she is well positioned, she has the potential to eliminate Roadhog. Additionally, Fortify makes her immune to Chain Hook.
Reaper deals devastating damage up close, and Roadhog has a large hit box, which means several shots should finish him off. However, If Reaper gets hooked and takes enough damage, he will be at a major disadvantage if he doesn't die.
A 1v1 against another Roadhog is an interesting battle. Whoever hooks the other one first will have a major disadvantage if both are at full health (because the one who hooked second has more time to plan and can hook back), but if both are at roughly half of their health, the one who hooks first will win.
Successfully landing Chain Hook on Tracer will guarantee a free kill since she has low health. He can use his Take a Breather ability to regain any lost health. However, if Roadhog misses his Chain Hook, Tracer can use him as a farm for ultimate charge, as his hitbox is massive and he may not be able to do much against an unstunned Tracer.
Zarya can use allies hooked by Roadhog to easily gain energy via her Projected Barrier, and at high energy, will be able to win against Roadhog when his Chain Hook is available, even if he uses Take A Breather. Zarya's personal barrier can be used to block the hook, and one shot is normally enough to give the entire 40 energy, however, if Roadhog hooks Zarya while her barrier is unavailable, he will have the upper hand provided he shots his primary fire at her.
Roadhog is a particularly versatile tank with a rather selfish playstyle, meaning he can easily play many roles without the need for much team support. However, Roadhog's biggest strength is playing aggressively, as he is not like other tanks, who are armed with durable barriers. Giving him a higher damage output with certain abilities can turn the tide of an unbalanced 5v6.
Roadhog's large hitbox, high health, and tendency to take large amounts of damage make it easy for Ana to quickly gain ultimate charge. Ana can Nano Boost Whole Hog, which can be a fearsome combination when used right.
Roadhog's ability to hook targets allows Mei to use her Ice Wall to isolate them more easily. In the event that Mei splits off two enemies instead of one, and Roadhog is the only one nearby, he can easily take care of one of them while she freezes and kills the other. Additionally, her freezing makes it so Roadhog can save his hook for a different target, if need be.
Roadhog can deal tremendous damage with his Scrap Gun or Whole Hog if given a damage boost. His Chain Hook, high health, and advantage to Mercy's common threats deters them from attacking her if he's nearby.
Because both Roadhog and Soldier: 76 have a self-healing ability, they can split off from the rest of their team without much pressure to flank the enemy team and deal high damage from an off-angle. If Roadhog fails to kill a target with his Chain Hook, Soldier: 76 can follow up and secure the kill. Additionally, Chain Hook can temporarily stop Defense Matrix if it lands successfully, and gives both time to get D.Va out of her mech so they can run wild.