Zacian is better
Here's why (for their respective "Crowned" formes):
- Zacian Crown has fewer weaknesses (2) and more resistances (9) than Zamazenta Crown (3 and 8). While Zamazenta's defenses maybe higher, it is also more likely to be hit by neutral or super effective moves, so it is easier to knock out over the course of a few moves.
- Zacian is faster. Zacian's base 148 Speed allows it to outspeed some common threats like Dragapult, Galarian Darmanitran (Zen) and Zeraora. If/When more Pokemon become part of the Sw/Sh Ubers pool (post DLC release) this gap will only widen. Zacian Crown is faster than Ash-Gren, Mega Mewtwo Y and many others (should they be released). This clearly puts Crown Zacian at an advantage.
- Zamazenta's lack of access to Body Press, arguably one of the best STAB moves with it's monstrous defense stat makes it rely on Close Combat for firepower, which cannot be as easily spammed. While Zacian doesn't fare particularly well either, that requirement for firepower is toned down due to its insane 170 Base Attack.
- Zacian has access to Swords Dance which immediately puts its Attack at +2.5 (due to its ability), making it fast, and deadly. Zacian can easily sweep late game if left unchecked. Zamazenta has no access to such stat boosting moves of that level of threat to the opposing team.
- Intrepid Sword is a much better ability than Dauntless Shield, effectively granting Zacian an immediate boost similar to Choice Band, but without the choice lock. The +1 Defense from Dauntless Shield is less impressive, since the meta game is clearly more offensively oriented than defense.
To sum, Zacian is faster, has better Attack, has access to Swords Dance, has fewer weaknesses, and is a much greater threat in general than Zamazenta, so it clearly wins out. Even 1-v-1, after a Swords Dance Zacian is a lot more likely to defeat Zamazenta than vice versa.
Note: This comparison is for their Crowned formes. However a very similar case can be made for their base formes as well, except for their Speed, which is a tie.