The aerial that ends first. You can use an aerial or airdodge to exit hitstun sooner than you can jump out of hitstun. You can fastfall as soon as you use an aerial and leave hitstun(but you can't fastfall when using an airdodge, which is why using an aerial > using an airdodge). You want to fastfall because fastfalling helps in canceling out your vertical momentum upwards. It doesn't actually matter what aerial you use for this though since you can start them all at the same time(the difference in when it "comes out" is for when the hitbox first appears, and that doesn't mean a thing in this situation). However, you'll likely want to jump or use a B-move for recovery and to fight against your momentum at the soonest possible time, so you want to use the fastest ending aerial you have. For Luigi that's his Fair.
And then it comes down to what moves counteract your momentum the best. Like in the video shown, some b-moves and even your second jumps will boost you *away* from the stage when you still have momentum from being hit. So even though a move like Fox's shine can let him stall his momentum in the air normally, it'll actually bounce him even further out if he tries to use it to stop his momentum after being hit. But some moves are better than others- G&W's bucket ALWAYS completely stops his momentum, both vertical and horizontal, and doesn't give him a boost away from the stage, so it's super effective.
I think for Luigi his green missile is a lot like squirtle's shell slide- it does a pretty decent job of fighting against your horizontal momentum without boosting you away.
So basically short answer,
Exit hitstun as soon as possible and fastfall with a fair(fastest ending aerial)
then green missile.
Everyone does this atm anyway...
oh, and DI as best as you can when you get hit by DIing perpendicular to the initial trajectory(most of the time that's up and towards the stage, or on vertical hits that means DIing horizontally.). This is pretty good for Luigi cause you just mash the A button while doing this and almost always do a fair this way and then you should input a fast fall as soon as you can. If you got hit horizontally outwards(think snake's ftilt) when you DIed perpendicular to it you put some of that knockback into a more vertical trajectory(cause you DIed both up and towards the stage) which you want to fight against with the fastfall, and fastfalling against a vertical knockback move is obvious.
I'm somewhat notorious for being really hard to KO in the group I play in, I used to play Samus in Melee where she was a tank, and Luigi has an incredible recovery this time around. Hardest move to DI properly, imho? Luigi's very own up-angled Fsmash, as habit to DI any strong knockback smash is to DI up and towards the stage, which in this case... just DIs you into an even straighter vertical trajectory for quicker death, where you really need to DI slightly down and away, which is counterintuitive. EX: A no DI snake doesn't get KOed until about 96% from up-angled Fsmash, but you can KO him off the top as early as ~86% if he DIs up and towards the stage, while living to maybe about up to ~105% if he was DIing the correct way. Funny thing is is that you need about the exact opposite DI if luigi uses his down-angled Fsmash.
