So I was lucky enough to pull Yunan over the summer, and he’s been my tank ever since I max’d him. He’s been great - my win percentage has skyrocketed since he took over. But now Grazul seems to be a hard counter - you can charge her up twice before Yunan has the mana to fire once and just ignore his otherwise devastating special.
Is Yunan done?
Not every player in the game was so lucky to get Grazul. Besides, it’s all about puzzle. If the opponent is lucky, can clear you out with a stack of any reds. If is not, not even Grazul’s grandma couldn’t save him from the Yunan’s fury 
Nearly every tank has a hard counter, e.g. Seshat counters Guinevere, Kiril counters Santa. And not everyone will have Grazul, and fewer still with her at 4/80.
Certainly having Grazul makes a Yunan tank less worrisome. That’s part of why I made sure I got her and why I’m busy leveling her. Regardless, he’s still a very solid tank choice.
There’s no reason for him not to be anymore even with Grazul. Every tank has a counter or a stack. Thats just the way the cookie crumbles. No hero is perfect.
I would say is the best. Passive tanks like Aegir or Kunchen don’t represent an issue, for me at least. But Yunan is not only as sturdy as the others. He’s lethal.
Pre-Grazul he was; I didn’t have good tools against him. Boss Wolf, Kunchen and Ursena (all purple…) are probably my three least-favorite after Yunan, and Yunan may still be worse than Kunchen.
For me, Yunan and Kunchen hold about the same level of cups, but Yunan’s win percentage is significantly higher. Super nice to scroll through 3 screens of enemy attacks and see all the green in there!