S imo should be
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, in no particular order.
Rosa's own dair is a large long-lasting disjoint that's great at challenging recovery moves while Luma's is a quick, high-BKB move that can set up edgeguards or get early kills edgeguarding/with rage.
Ryu's dair is fast, strong, has a great angle that lets you spike from slightly onstage, and combos off weak utilt (probably among other things). Definitely powerful and practical.
Cloud's disjoint, active frames, and autocancel speak for themselves.
Bayo dair's landing hit is hideously strong, though I don't think she can combo into it anymore which hurts it slightly. Still deserving of mention though imo, especially since it doesn't stall when used out of hitstun which lets you potentially use that early-killing landing hit as a combo breaker in some cases.
Kirby's dair is quite multipurpose. Hup canceling with the lengthy early autocancel, can be comboed out of including into kills with dsmash, lingers for years with a spike hitbox which is great for hitting 2frames among other edgeguard scenarios. Honestly an amazing move.
A tier candidates that come to mind include
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(safe shield pressure, combo starter),
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(low lag, combos/kill confirms on missed tech, good edgeguard tool), and
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(ridiculous active frames+disjoint, low landing lag+sh autocancel, triple turnip is death).
Lastly,
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dair shouldn't be higher than (but probably belongs in) B tier. While it has high potential damage and shreds shields, it has a high SDI multiplier that happens to make it punishable
on hit against opponents that know about it, catching someone in shield with it usually results in the last few hits poking for "ok" damage instead of netting you a shield break, and the FAF/landing lag aren't great if you happen to whiff. That said it's another lingering spike like Kirby's dair and it's great when it works, but it has flaws that aren't present for the better dairs.