Rufio
The Rufio is a famous Meta Knight combo, and is a staple of his metagame in both Super Smash Bros. Brawl and Super Smash Bros. 4, and is among the many reasons why he is considered a dominant character in both of his appearances (most infamously in Brawl where he is considered the best character by a considerable margin). The combo consists of Meta Knight chaining his Up aerial into itself multiple times (usually five or six depending on the opponent's weight, fall speed, and size, and the amount of jumps Meta Knight has left) dealing roughly 50% damage, and finally KOing the opponent if they fail to escape the finisher, which is commonly Mach Tornado or Shuttle Loop.
Overview
In Brawl
In Brawl, Meta Knight's up aerial is among the fastest moves in the game, with a the hitbox coming out on frame 2, and interruptibility on frame 13, meaning it can be performed three times within a short-hop. It also has a hugely disjointed range, low damage (6%), and weak knockback (40 base/125 growth). Along with his multiple jumps, all these traits make it a superb combo tool.
There are several ways to start this combo. Meta Knight's down tilt has deceptive range, and has a chance to trip opponents, in which case they are wide open to a dash attack, which deals weak vertical knockback and easily follows into his up aerial. Down throw can be used, but opponents can easily DI and air dodge against this. Neutral and forward aerials, with their favorable auto-cancel frames, can also lead into a dash attack at lower percentages.
Though this combo is devastating, players commonly opt to replace Mach Tornado with Shuttle Loop. This is because Shuttle Loop has quicker start-up and, when used in the air, semi-spikes opponents with powerful knockback. Also, Shuttle Loop transitions into a glide, making it much safer if whiffed than Mach Tornado. This variation is very similar to Captain Falcon's most well-known combo in Super Smash Bros., which involved chaining numerous up aerials into a high-altitude Falcon Dive, and has been dubbed the "Stairway to Heaven." In fact, some players refer to Meta Knight's up aerial combos with the same name.
In Smash 4
In Smash 4, the Rufio is still possible to perform, though it has received a few changes on how it is executed. However, due to the severe nerfs Meta Knight has received in the transition to Smash 4, the combo is much more pivotal to his metagame, to the point that his entire play style effectively revolves around it.
Meta Knight's up aerial is much slower, with a start-up of 5 frames, and interruptibility on frame 27, about twice as much as in Brawl. However, it deals 1% less damage (6% → 5%) and weaker base knockback (40 → 30), but more knockback growth (125 → 128), and even with his decreased range, the move still has deceptively large reach (though it is very difficult to land on grounded opponents). Meta Knight's increased air speed also makes it more difficult to DI horizontally out of his up aerial, combined with the removal of hitstun cancelling.
Like his previous iteration, Meta Knight has some reliable set-ups into this combo. Dash attack deals more knockback (40 base/88 growth → 70 base/90 growth, and is now disjointed). Down tilt has shorter reach, but is still quick and safe, and can trip opponents. With the aforementioned removal of hitstun cancelling, both down and forward throws can lead into the combo, although the latter is escapable via DI, and the former is completely guaranteed only against heavyweights and fast-fallers.
Mach Tornado deals noticeably more knockback than in Brawl, but is now mostly forgone in favour of Shuttle Loop. While Shuttle Loop is much riskier to use since it no longer initiates a glide, it deals more damage and has much higher knockback growth, and the aerial version deals vertical knockback instead of horizontal, making it much deadlier when used near the upper blast line.
It is worth mentioning that this combo can be used to zero-death many opponents by exploiting the changes to Meta Knight's back aerial. His back aerial now uses the autolink angle. As such, Meta Knight can dash attack and follow-up with a back aerial, and then fast-fall and land before the final hit. If the opponent does not tech, he can jab lock them with his down tilt. As they are forced into their neutral get-up, he can land a dash attack, grab, or up tilt, and then start the up aerial chain, and finish with a Shuttle Loop, KOing if executed properly.
Alternatively, Meta Knight can dash attack into an up aerial and then footstool the opponent, and in this case, they have no opportunity to tech, making it a completely inescapable zero-death. Interestingly, this zero-death works on all characters except for the heavyweights, such as Donkey Kong. After the back aerial, they only flinch rather than fall over, and when footstooled, Meta Knight cannot reach the ground fast enough to start the jab lock.