Critical Hit (also referred to as Critical-hit capability) is a bonus effect in Super Smash Bros. 4 and Super Smash Bros. Ultimate. In Smash 4, it was seen as one of the strongest bonuses available, and was eventually banned alongside Explosive perfect shield.
In Super Smash Bros. 4
Critical-hit capability was one of the most common bonus effects in the game. It gave its user a 20% chance of dealing a critical hit with 3x damage and knockback. As a result, matches quickly became a roll of the dice where the winner would be the one that got a critical hit first. The community unanimously banned Critical-hit capability in 2016.
In Super Smash Bros. Ultimate
Critical hits were drastically nerfed in Super Smash Bros. Ultimate. Its 20% activation chance has been lowered to 8%, and when a critical hit does land, its increased damage and knockback is almost unnoticeable. Another variant, Critical Hit ↑↑, was introduced as well. Critical Hit ↑↑ boosts the 8% chance to 12%, but is functionally identical otherwise.