That I rather play against and Ana than a Zen when playing tank? 
Na it’s a totally valid opinion. I dislike being discord orb’d on tank too, even if I personally dislike being naded more
I rather deal with an Ana than a Zen. I dont see Ana as big of an issue as most players on this game.
I think it depends what tank you’re on. Anti is much more effective on some than others.
He does 312.5 DPS before damage reduction and amps damage against you by 25%. I would argue an unchecked Zenyatta is the most lethal hero in the game for a tank.
That said, he has no mobility, no sustain, and his only crowd control is his quick melee. He’s an easy target for a lot of heroes.
I get that.
If you can bait or block the sleep/nade, you have a bit of a pause before the next one(s) will trouble you.
With Zen, if he is so determined, can keep sticking you with the debuff over and over again, with little chance to avoid it. And getting it removed (generally) means you have to remove yourself from the front line fight.
They are both (one of) the proverbial tank busters in a match…IMHO…and you as a tank (depending on your choice of tank) rely on your team to deal with them, because a smart Ana or Zen are not going to let you get close.