Joanna by a mile.
Holly's boxing is very limited, hell, she doesn't even have a respectable jab. Let that sink in. Her punching form is
pretty terrible and her setups & combos are limited and quite predictable, there's a reason that Valentina was able to check hook her in the face every time she stepped in with punches. Look at every single Holly Holm fight including her boxing matches, nearly every sequence is foot feint, foot feint, then throw the left straight and blitz in with straight right-left punches. She doesn't work off a jab, feint with the lead hand, double up on punches, or anything like that. Her best punch is her right check hook which she pretty much never throws because she has no way to set it up unless someone runs at her like Ronda.
Joanna has a far better jab and tons of setups off of the jab. She'll change target & timing with the jab, setup left hooks or right hands off the jab, adjust her punches to hit open targets, and she's effective on the lead or the counter. Her punches are also a lot crisper than Holly's and thrown with proper form, not to mention she's at the right distance to actually land them instead of punching air.
Put it this way. If I didn't know anything about the fighters and you told me there was a former boxing champ at WBW I'd never guess it was Holly. I'd guess that it was Nunes since she's the one who's knocking everyone senseless with punches, and actually punches with good form and power. Nunes, Shevchenko, de Randamie, and hell, even Pennington have better boxing than Holly.