Can Corrin's counter whiff/miss the opponent like Shulk's and Little Mac's? It seems pretty reliable AND powerful. Near the ceiling it's crazy. I'd say it has a decent chance of being the best.
The hitbox is terrible underneath Corrin. So for character with large, disjointed Up-Airs, they can juggle Corrin without fear of being countered (when spaced).
All the Fire emblem characters in particular are good at this. Same for the Links, Falcon, and ZSS.
Be careful anyway because, as everyone knows now, Corrin's counter is something that must be respected at all times.
Oh, be really careful with multi-hit moves too as some of them have hitboxes with high knockback. I countered a Falco FAir (not even the last hit, just a random hit in the middle) and the Falco died before 100%.
EDIT: I should probably put my opinion in here too. At a low level, I'd say Corrin's counter is debatably the best in the game, since it can KO at ludicrous percents.
However, at a high/top level, Witch Time is by far the best counter in the game. It gives Bayonetta an easy setup for her plethora of 0-deaths, and has much better frame data (FAF is 45 compared to most counters which have a FAF of at least 60). Add in the fact that, if fairly fresh the opponent is frozen for about two seconds, and it's debatably better even at lower level play due to it still being able to lead to ~40% punish at low %s and death past 60.