Hanzo has to be the best counter for Orisa not only for his annoying arrows, but his ultimate that sends Orisa out of defense. As an Orisa player, I can tell you that Hanzo is a frustrating, and dangerous hero to counter Orisa.
Not only are his arrows annoying to defend against with the Javelin spin and energy javelin, but Orisa has no where to go to have relief from the damage. As Orisa is a very defensive tank, Hanzo can wipe her out quite easily with his ultimate and precision, forcing you to failure. If you have a hard time with this hero, learning how to counter Hanzo will greatly increase your odds of winning!
Cassidy is another one of the best damage heroes that Orisa counters really poorly. Since Orisa's Terra Surge can take up a decent amount of time, the high damage I have received from a Cassidy player can immediately end my Ultimate really quick.
Since Cassidy has such a high damage rate with his primary fire, this makes Orisa a prime target to overwhelm her with aggressive fire. The Javelin throw has been my best counter against Cassidy, keeping him behind the front lines and away from Orisa's frontal push.
As a Lucio main, I have found countering Orisa super easy, and even make the player rage quit if it gets too heated. Specifically in control maps with possible drop offs (Like Ilios), I can easily use Lucio's shock wave to push Orisa off the grid. Since Orisa is rather light compared to other tanks, this makes Lucios abilities fun to bully her with.
Not only does Lucio have great offensive measures to use against Orisa, she just cant get her Javelin throw on him, which makes him almost impossible to keep an eye on 1v1.
Symmetra has a turret that completely blows Orisa's defense matrix out of the equation. Since Orisa has only a specific amount of time to provide herself health and defenses, the turret keeps her too overwhelmed, which has killed me in the past SO many times using Orisa as a tank hero.
If you want to counter Symmetra, Orisa is not your best option. From experience, I simply cannot shake her abilities unless I have really good damage and good support heroes helping me push through. This requires all the heroes to work together, and that can be difficult if your team cant group up.
Junkrat's Ultimate can completely ruin an Orisa player, and it has for me. Due to his ultimate, even if I am using Orisa's Javelin spin or even Fortify, she is just not the best counter for Junkrat.
If you want to easily win against Orisa, Junkrat will for sure send you and your teammates to an easy victory. Orisa as an enemy tank can be really frustrating with her defensive tactics, but I know for a fact she has no chance against a good Junkrat player.
As if Blizzard entertainment couldn't make an Orisa player even more annoyed, the new updates on Roadhog have made him a frustrating hero to counter Orisa. I say this an Orisa player, that if his Whole Hog is all on you, you're ruined.
Other than his ultimate, though, his close range primary fire has a pretty high damage, pushing its way through Orisas defense, I have found aggressive Hog players are super annoying to counter with Orisa. So if you want a good tank hero to counter Orisa, Roadhog is an excellent option.
Doomfist has a charge that can send Orisa into major defeat, making him for sure one of the best counter for Orisa Overwatch as seen so far.
Although I don't come along many players with Doomfist, he really has been one of the most challenging characters to face on the enemy team. When going against Orisa, he has cancelled my terra surge countless times with his high damage output.
Although not many believe in Zarya players, I promise a good one can send Orisa to respawn faster than any other tank hero. Zarya has a high damage heavy weapon that simply destroys any attempt I have to defend against her.
If Zarya is using her ultimate, you might as well bid farewell to victory, as she can melt through an Orisa player extremely fast without good support. If you want to counter Zarya with Orisa, I have been able to push her out of my way easily if I have a good Mercy player by my side.