I would personally give it to Lucario's or Shulk's counter.
's counter got its counter activation window drastically reduced from SSB4, as well as its intangibility frames. This offsets the slightly reduced endlag, as it is now hard to land the counter and it is now easier to punish him out of it despite the lower overall duration. The move is also only strong at higher levels of aura, although he does hit like a freight train when it comes to that point.
's counter is the paper-definition of unreliable. Despite the move's overall bigger hitboxes from SSB4, the counterattack often whiffs, especially in the air, due to the slowdown effect not being strong enough while Shulk succumbs to the game's physics during counterattack. Using it offstage is often a death sentence for Shulk, even when he lands the counter. Despite the recent buff to his counter's power, the move is still slightly weaker (with no arts) than its SSB4 appearance. When the stars align, and he hits a Smash Art fueled counter, it will hurt hard. However, instance of this is very rare, and Shulk players hardly ever go for the counter due to how unreliable it is.
The counter also has some odd/rare interactions, where Shulk can sometimes get hit during the counterattack, especially by Rosalina & Luma, due to the intangibility frames not properly covering the entire counter. The normal counter, for example, has its intangibility ending when the counterattack hitbox comes out, which btw, can be blocked in Ultimate (it was unblockable in SSB4).
Honorable mentions for unimpressive counters:
's physical counter still has some very notable issues despite the buffs it has received from SSB4. The biggest of which is the hitbox, as the counter hitbox is still very slim for a counterattack, which means that it can sometimes whiff on low profiled opponents, and especially on aerial opponents similar to Shulk's counter (although not as severe). The move also has an awkward sourspot/sweetspot properties, with the sourspot being the area just outside the staff. This weakness was more prominent in SSB4 when the sourspot was comically weak, but it is still something regardless. Still way better than the trash it was in the previous game.
's counter hits hard. However, it is also arguably the slowest counter in the game, even slower than Witch Time. It has a 1.2x damage multiplier like Marthcina's, but it is also a lot slower in terms of endlag, a whopping 74 frames of endlag.
's is interesting. Despite the buffs from patch 3.1, her counter is still unreliable at times. For starters, the startup of her counter is the slowest in the entire game, starting up at frame 8. It is technically tied with Chroy for the slowest counter, but Chroy's counter intangibility starts at frame 7. Bayo is not intangible until frame 8, so there are moments where it seems like you landed the counterattack, but Bayo gets hit anyways. When she does land the counter, there are moments where she doesn't get much out of it due to it's low duration. Multi-hit moves are also a pain to deal with this move.
's is not the worst thing in the world, but the risk-reward ratio is not very good. For starters, it has the shortest counterattack window in the game, only being 20 frames. The KO power of this move got heavily nerfed from SSB4, both directly and indirectly. The end result is that this counter is surprisingly weak for a counter. The main advantage is that it sends opponents upwards, which is where Corrin wants the opponent to be. However, with its much higher endlag after counterattack, being 67 frames, Corrin can't really take advantage of this anymore outside of very specific situations.
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's counter, while flawed, has some pretty good utility. For starters both the shield and especially the counterattack has surprisingly big hitboxes. While the fact that the counterattack triggers by itself makes it one of the more punishable counters when whiffed, the fact that it still emits a hitbox can oftentimes trip opponents up. It is also one of the strongest counters in the game, having the damage multiplier range from 1.2x-1.5x, and it can be extremely powerful knockback. I had an instance where countering Little Mac's side B would KO him at 60-ish% at near-center stage. I heard that countering certain stronger Smash attacks (not strong enough to break it) has a chance to KO at 0%.
The move is also especially helpful in edgeguarding, as upon successful counter, it stuns the opponents longer than the average counter, making it nearly impossible to dodge the counterattack upon successful counter.
A funny thing to note, is that the counterattack, similar to Peach's Toad counter, is considered a projectile. It can be both reflected and absorbed. While instance of reflecting/absorbing his counter is very rare, it is a funny interaction when it does happen.
