Which unit is the best debuffer?

It really depends, honestly. There's Arvis which can ploy all 4 stats, but that only happens when they're in the same column.

There's someone like Soren that inflicts a Chill effect on someone with the highest stat of said Chill.

I think Lilina might have a Chill effect in her tome.

Arvis and Saias are the only units who can run all 4 ploy skills simultaneously, Lute can run three and is general better than the both of them.
Spring Catria and Sharena have solid chill debuffs in their 'tomes' however be sure to keep them healthy (I use the former often in place of Rein, her stat coverage is very nice and if you built off her res a bit she can potentially use ploys though Sharena can do this much easier).
Lilina can run a B skill chill like how Soren can, and Marth could easily too since his B skill is usually pretty open. Other units with open B slots would be Sanaki, Reinhardt, and FS bow users.
I also have a ploy based Joshua who can proc them pretty consistently even without Fury. Staves like Fear and Slow are good with Dazzling refine, and among the best Dagger debuffers are anyone with a Smoke Dagger and Sothe because of Peshkatz.
Shout outs to Gunnthra but her res is too low for Ploys imo

Soren have Atk chill, Lilina have Res chill, Saias can debuff 4 stats like arvis, but arvis have more edges with his B slot and can be a support healer too. Gunnthrà debuff 6spd and atk to the unit with the lowest def

I personnally use Arvis in my main arena team and he is really amazing.He have my summoner support and debuff every stats. He debuff, heal, destroy almost any green (and more if they are debuffed) and have very good matchup vs red too

You could run Arvis or saias who can run 4 ploys. Gunnthra with two ploys and chilling seal. There’s sothe who can debuff all stats by 4 after combat and buff himself by 4. If you have inheritance you could inherit some chills to characters. The heaviest debuffer would probably be gunnthra with her chilling seal double ploys but ploys are unreliable and require placement to be precise. All in all it depends on what kind of debuffer you need and how it suits your playstyle

Arvis is a pretty danged good multi-purpose mage with access to three different ploys and since he heals every turn he can make very good use of the easy-to-get Fury to make himself quite dangerous to approach. The sucky part is that he doesn't have Speed Ploy in his native kit, and unfortunately the only unit I have who does is Saias. He doubles quite a bit, hits hard (most of the time), and can provide backup via Ardent Sacrifice if necessary since he'll get the health back.
Although I don't really think there is any one debuffer who's good in all fields; they all struggle somewhere.

Arvis and Gunnthra are two of the best debuffers imo.
But Saias or units with double chill are very good too.
And theres FTakumi with a niche, but amazing weapon.

Look to the picture, you can use 2 at the same time for even stronger debuffs.

What about Sothe. He can debuff the enemies and buff his allies at the same time by 4 points.

If you give him windsweep he attack for free.

If you enjoy using Marth, Arvis plays a very similar role but as a debuffer instead. While Marth buffs everyone's stats near him, you can ploy all 4 stats with Arvis. You also get to keep the combat medic type thing going but instead of Falchion +renewal /ardent sacrifice you can use recover ring/ardent sacrifice for essentially free heals every turn. Otherwise, smoke dagger on saizo is really fun to use. -6 to all stats is crazy, but harder to make use of than ploys imo.

Oh right saias is also basically blue arvis and it's easier to get all the ploy skills on him so he's also good for that.