Who should I feed my +spd -atk Hardin to?

Any brave weapon user is a prime candidate for it since it basically makes them quad everything when initiating. (some find it overkill, I find it satisfactory) Brave Lance Effie, Brave Axe Chrom, Brave Bow Jakob are some examples. Some armors can also run Bold Fighter with Steady/Warding Breath +QR Seal to have follow ups and cooldown reduction in both phases. Some examples are Grima and BK/Zelgius. (basically armors with innate DC in their weapons)

BK/Zelgius would be optimal choices. If you ran one with steady breath/bold fighter and the QR seal, you could double on all phases no matter what.

Grima would absolutely love either Brazen Def/Res or Bold Fighter, but if I manage to get Hardin I'm personally going to put Brazen Def/Res on my Lissa and put Martyr+ back on her (she actually is the best healer to put it on, because her defenses are pretty solid and balanced at 28/30).
Sure, Azama has 4 more Def (32), but he also has 5 less Res (25); sure, pretty much every other healer has a few more Res than she does (8/14, with 3 additional healers tied with her), but she also has the most Res out of each of them (Wrys has the highest of them at 22, which is 6 less, while Sakura at 30 Res has 24 Def).
Seriously, I'm considering requesting Lu Bu to add Lissa to the Paths of Pain+
EDIT: To be honest, though, any unit with balanced defenses that doesn't have a problem with either range is definitely worthy of getting Brazen Def/Res. Winter!Lissa and Winter!Tharja, other than himself, have the highest balanced defenses in the game (though I could argue a case for Fjorm, Gwendolyn, and Sheena if you want to put it on a more common unit). Halloween!Jakob is also probably THE best bow-user to use it, as long as you give him Guard Bow+ and a Distant Def seal. If you're trying to stray away from Armor units, Camilla and Myrrh are also great ideas (considering they also has access to Flier Emblem buffs). Selena is a great Infantry option who is known for her really high Def and Res, though her Atk is pretty low which will require much assistance and inheritance.

As some others have pointed out, I'd say your best bets are BK/Zelgius or Grima. Personally, I used my Hardin to give Grima Bold Fighter and now he's a monster. If you have any merges on Zelgius, he's probably an even better option given how powerful Black Luna is. My Grima is included below. Im currently building his Support levels.

Edit: Winter Chrom also seems like an excellent choice but I don't have any personal experience with him.