Bleach is honestly just getting better and better. Among the big 3, we saw Yama's Bankai first, then Unohana's, though it wasn't explained very well, and finally Shunsui's Bankai. To think that we would also have other characters releasing their bankais in the future, like Urahara, maybe Aizen as well as the members of the Royal Guard.
For Shunsui's bankai, though, I really am wondering if it is stronger than Yama's bankai? Because, honestly, Yama's Bankai can actually be countered, in some sense. Like, the few ways that I can think of are closely related to pressuring Yamamoto to use bankai for an extended period of time, which would undoubtedly kill him, dodging any attacks while keeping a distance and perhaps even ultra strong barriers.
For Shunsui's bankai, I really cannot find any way in which it can actually be countered. Like, honestly, there's absolutely no way that one can avoid getting caught in his bankai. I mean, if you count intangibility, which is how Lille countered it but we're talking about a situation where the opponent is subjected to physical damage.
I really don't see any reason why his Bankai isn't considered the ultimate bankai, just because it can't be countered in any proper way. I mean, let's just suppose that you did have Yamamoto himself, fighting against Shunsui. Even with the protection which Zanka No Tachi grants him, he still can't avoid being affected by Acts 1,2 and 3. So, I don't know if all of these things actually make Shunsui's bankai the ultimate bankai.
Note: The Ultimate Bankai, in this case, isn't referring to the strongest bankai. The Ultimate Bankai is just referring to a Bankai which cannot be countered in any comprehensible way and, therefore, can be used in every single battle, without the bankai itself being at any disadvantage. The strongest bankai, I think, would be one which can compress all of the power of the bankai into a tiny weapon, which can then be used for offensive purposes. I mean, that's why Yamamoto's bankai was so strong and why Ichigo, despite being considerably less experienced and, frankly, weaker than many opponents, managed to beat quite a number of them, just because his bankai being small and compressed gave him a huge amount of strength and very high versatility. That's just because he relied mainly on swordplay and therefore, his former bankai gave him a huge advantage, playing on his strengths.
The problem with his Bankai is that its used as a last resort, unlike other bankai's Shunsui can only safely use his if non of his allies are near. Yama while very destructive, his allies only need to not get his by his attacks to stay safe, but Shunsui allies need to be no where near him when he uses his Bankai. Too add on to that, Yama's Bankai which if used for too long will kill him, Shunsui can be killed by his Bankai at any time. His bankai gives the enemy 4 acts before finally dying, if the enemy can beat Shunsui in the first 3 acts before the 4th then they win, while Yama's bankai is about brute strength and the only way to beat it is with equal strength or skill, the way to beat Shunsui is to do the one thing his Bankai is all about, play the game to win.
Zanka no Tachi isn't safe for others if you aren't hit by its attacks.
Royd said that if it wasn't for his Blut Vene then he would of burnt to ashes just being near Yamamoto, and even from far away it still immediately evaporated all the moisture in Seireitei just by being activated, and melted Toshiro's Bankai which is considered the strongest Ice Type.
I personally think that you either have to hold an outrageous amount of reiatsu on the level of either Dangai Ichigo or Hogyoku Aizen, or be at least Captain Class with an exceptionally powerful defensive technique like Heirro or Blut Vene.
Shunsui's Bankai is definitely powerful, and can be used to injure foes that can't otherwise be injured such as the Intangible Mega Ultra Chicken Lille to a great degree but it doesn't seem to have a nearly unbeatable defense like Yamamoto's sun shield, merely a deterrence from attacking him as the wound might be reflected on you as well.
In addition Shunsui made a point to think that he couldn't use it around his allies as his Bankai will affect anyone within it's area of influence, so it isn't exactly a safe Bankai to pull out when surrounded by your allies.