Up Smash is the usual go-to. It's a devastating anti-air that can deal a huge amount of damage and has low enough base knockback to combo it into aerials of your own, and the late hit will usually frame-trap them. The only real downside is that you will want it fresh to close out a stock, so try not to use it at mid percents until you are sure it will get the KO.
Up Tilt is faster and safer on whiff, but has less disjointed range and the hitbox is much harder to land. In Ultimate, they FINNALLY removed the hitbox of the tonge, turning it into Greninja's most disjointed move, even more than the kunai attacks. Thank Arceus.
Substitute and Shadow Sneak are fantastic punish tools for hard reads in general and will leave them in a really good spot for an edgeguard or a juggle, so using them to catch a landing aerial at low/mid percents is absolutely recommended if you are confident enough to commit.
Pivot Grab has a massive range and can work, especially if you want to close out the stock near the ledge, but it's almost as slow as Up Smash.
If you are away from him, a fully charged Water Shuriken is your best bet. They will likely land in time, but it will keep them in shield long enough for you to safely punish with a grab.