I know that I don't keep track of the competitive scene, but I'll just bring up my own comments on some of the custom special moves.
Fast Fireball: Less powerful than Fireball, but it's easier to spam with.
Scalding F.L.U.D.D.: More useful offensively, despite having less range than F.L.U.D.D..
Hot Slap: Being able to hit airborne targets feels good, especially when the move deals more damage as well.
Boomerang: More effective on offense without the risk of bringing anyone towards you.
Whirling Leap: Greatly improves Link's recovery, though you can't deal any damage with it.
Egg Launch: Although opponents can break free from the egg quicker, this move can prove to be a big threat for any fighter whose horizontal recovery is very poor.
Upper Cutter: Strikes faster, and while it does leave Kirby helpless, at least it doesn't mess up his horizontal recovery.
Quick Feet: Basically designed for those who have trouble with Quick Attack. The increased distance helps as well.
Lasting PK Thunder: Easier to control, and can't be gimped so easily either, which can help Ness's recovery more.
Fire Shot: A bit more convenient on offense, as it doesn't weaken over time.
Dark Fists: Much more effective on offense, and the longer start-up time offers a better horizontal reach too.
Wizard's Dropkick: Improves Ganondorf's horizontal recovery, despite being weaker, and lacking any meteor smash properties.
Dreadful Tornado: While this move does deal less damage overall, its knockback is not to be underestimated. Just beware of the ending lag increase in the end.
Blade Coaster: The move can still be used offensively, and offers better horizontal distance.
Rose-Scented Waft: Charges faster, and while it does deal less knockback, adding flowers onto your opponent's head can help Wario finish off his adversary more easily.
Aether Drive: This move's increased horizontal distance can mean that you can try mixing up your recovery between using Quick Draw or Aether Drive.
Dragon Rush: No need to worry about damage penalties with this move, and it can't be stopped prematurely either.
Fly High: Although it can't deal any damage, the increased vertical distance is nice for recovering low.
Winged Pikmin Jump: Better vertical recovery in the sense that you don't have to worry about your Pikmin supply weakening the ascent.
Balloon High Jump: Better vertical recovery, especially when recovering low.
Luma Warp: Can't be reflected, and enables Rosalina to put the Luma at a desired spot quicker.
Shooting Star Bit: Travels quite far, and very handy for putting pressure from a distance.
Guardian Luma: Designed for fighters who lack projectiles, the move can help keep their approach options at bay.
Stunning Straight Lunge: Despite not offering knockback resistance, the move can at least travel far without having to be fully charged.
Guard Breaker: While this move is not good in terms of recovery, being unblockable means that you can deliver a big blow without any fear of the attack getting blocked or countered.
High-Capacity Pump: Albeit being slower, the travel distance increase is always welcome.
Exploding Attack: Although this move does not counter, it's an improvement over a counterattack that lacks any invincibility frames.
Angelic Missile: Helps Palutena's horizontal recovery quite well without having to use any mid-air jumps.
Piercing Cannon: Although it deals less damage and knockback, the move fires faster to make up for it.
Koopa Drift: While the move is less powerful, being able to deliver multiple weak hits during the spinout at least makes it safer to use. It also improves Bowser Jr.'s horizontal recovery quite well.
Major Ejection: While the decreased vertical distance may be inconvenient for recovery, the added meteor smash property means that you could potentially gimp an opponent's recovery if you time the attack just right.
Impatient Mechakoopa: More designed to be a projectile, but it can potentially be used to juggle opponents around, and since it explodes upon landing, opponents can't really use the Mechakoopa against Bowser Jr. without possessing a reflecting move.
High-Explosive Shot: A bit more convenient as a projectile in the sense that it travels farther, and you have more control over when the can explodes.
Again, I can't exactly say how good the moves are competitively.