Marth (SSBM)/Down aerial
Marth's down air is considered to be the second best spike in the game behind Falco's. Feared for its deadly and consistent ability to take stocks, the move is a staple move in Marth's metagame and is the go-to finisher for many of Marth's aerial combos. The simplest and most well known of setup is the Ken Combo. A forward aerial knocks the opponent upwards, leading into a jump, and then a spike with this move.
Despite having many setups, the move is still just as deadly on its own. Its wide range and disjoint allows it to cover most low recovery options; with good timing, players can use a ledge hop down air to catch opponents recklessly jumping or recovering back to the stage for a rather efficient KO. The move also has many uses that don't involve edgeguarding. The move is very fast and with its reach can be used to setup combos or tech chase in a similar manner to Falco's down aerial or Captain Falcon's down aerial. The move can even be used to out of shield to punish cross-ups to great effect. Interestingly, unlike the majority of Marth's moves, this move's tipper hitbox takes priority over the others' should they both collide with the opponent. This means that landing the tipper with this move is relatively much easier compared to his other moves.
The move is not without its drawbacks, however. While Marth's dair is nearly as fast and powerful as Falco's and has much more reach, its duration isn't nearly as long and has a sourspot. This means there aren't nearly as many situations where it can be landed on its own without proper timing. In addition, the move has much more landing lag, making it much more punishable on whiff.
In the PAL version of Melee the move is nerfed so that it is now a meteor smash instead of a true spike. This means because of its rather low hitlag compared to other meteor smashes, it is relatively easy to meteor cancel on reaction. However, as a trade off, the more vertical angle in PAL allows the move to combo and setup better on stage. Regardless of its drawbacks though, the move plays arguably the most important role in Marth's game as his go-to offstage finisher and edgeguarding move and is very much needed in matchups where stocks must be taken as efficiently as possible, especially in his matchups against the space animals.