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 2 seconds of Whole Hog, meaning he is very vulnerable should he decide to use it 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.
Reaper's Hellfire Shotguns' damage output combined with his passive ability make him a very effective counter against Roadhog when it comes to close-range. His Hook and Scrap Gun combo won't deal enough damage to kill Reaper outright and he can simply keep regaining health through The Reaping which will help him survive the rest of Roadhog's onslaught and eventually kill him. The only sure way for Roadhog to kill a full-health Reaper is to hook him off the map or push him off using Whole Hog. Additionally, he can use his Wraith Form to prevent getting hooked altogether and/or escape Whole Hog. Reaper can also benefit more from Roadhog's Take a Breather than Roadhog himself, as he will be unable to attack and Reaper's passive will allow him be healed as well.
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.
Widowmaker's hitscan weapon and extremely long range means she can stay well out of Roadhog's limited range. Additionally, due to Roadhog's unusually large head hitbox, it is much easier to land critical hits.
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 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.
Since Roadhog's Hook and Scrap Gun combo won't be enough to kill Mei outright, she can quickly enter her Cryo-Freeze and regain any lost health. She can then blast Roadhog with her Endothermic Blaster which will create enough distance between them and eventually freeze him to the point where he will be in a critical state and will have to use his Take a Breather ability. Mei can then capitalize and continue freezing him which will cancel his Take a Breather ability potentially nullifying any healing he may have done. From this point, Roadhog can still potentially deal enough damage to Mei to force her to use her Ice Wall or escape using another method or she can continue the assault and eventually kill him with her Icicles.
McCree's Peacekeeper will deal massive damage to Roadhog from a distance as well as close range with Fan the Hammer. Since Roadhog is limited to close range duels, he will have trouble taking on McCree from a distance. He can also use his Combat Roll to get close and Flashbang him while he is in stasis to interrupt Roadhog's Take a Breather ability and capitalize on him using his Fan the Hammer. However, if he is hooked he will be at a disadvantage. The Hook and Scrap Gun combo won't deal enough damage to kill him outright, but he will be in a critical state regardless.
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 BoostWhole Hog, which can be a fearsome combination when used right. Additionally, her Biotic Grenade increases Take a Breather's healing to 450.
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.
Zenyatta can apply an Orb of Discord to any enemy that Roadhog hooks, further amplifying the devastating damage caused by the hook/scrap gun combo. Additionally, Zenyatta can easily leave an Orb of Harmony on Roadhog for a while, and gain a lot of ultimate charge because of Roadhog's large health pool.