Hoping this gets a damage buff.
Oh no. Luigi's F-tilt does 8%, and there is some sort of hidden rule in both bros, for moves that Luigi cloned from Mario, Mario can
never get a damage output bigger than Luigi. It must at least be the same or below
.
Kidding. Hoping that (And Luigi's) get some damage buffs too, but it's rather unlikely.
Anyway, U-smash, as others have said, is your best anti-air tool. It does 14% (Somewhat big compared to D-smash, don't you think?) and sends enemies perfectly up, meaning DI will has less chance on saving their butt. It also hits on F9 from behind, and F12 in the front, so it's fairly quick. Though, the range is rather small, so you don't want to clash with swords or something here.
Best killing tool Mario has overall, and is also a good punishing tool and OoS option. Though, as usual, don't spam it. You'll just get your butt kicked, and be way more unpredictable (No arguments on how good it is though).