Gravitational Pull shuts down projectiles and item projectiles a lot of the time. With customs on, Guardian Luma is like an all purpose counter move, and Rosalina can still draw in item projectiles (like Peach's turnips or Mega Man's Metal Blade).
U-Tilt/U-air shuts down a lot of overhead approaches.
D-air can shut down underside approaches but the timing is kinda hard. You need to really read the opponent to get them at the later end of the animation.
Dash Attack cuts through and clanks with tons of moves. As does F-air and N-air. B-air, N-air, and F-air also double as dodge maneuvers meaning you can reliably bait and punish with them. Also Rosalina's F-air can go right through a lot of projectiles (almost as if they are getting sucked into the galaxy she creates during the move) I have noticed, generally smaller ones or uncharged ones.
Those are specifically with just Rosalina, though these moves are obviously even better with Luma. Luma's Dash Attack does not share the high priority of Rosalina's though.
When taking Luma into account, his spinning rapid jab beats a ton of moves, and is really useful for stage control if you know the "de-synced Luma Spin" trick. Also his D-air is better for clanks and stuff than Rosalina's since you don't need the late hitbox.
Luma's Star Bits are also great for stopping people in their tracks as a quick little poke move that can often be followed up on, and with customs on, Shooting Star Bit is basically Falco lasers but better because I believe they are faster and the Star Bits are a bit wider than his lasers.
Luma Shot, when fully charged, makes Luma almost completely invincible. With customs on, Luma Warp can be used to send Luma right on top of opponents in virtually no time at all and is exceptional for interrupting attacks and countering with something strong like a Smash Attack.
Also just learning all of the little nuances of manipulating and controlling Luma will make you a real annoyance to your opponents.
There are a ton of other things but I am making a Mario Maker level right now...
