Regardless of where you have 58-year-old Whitebeard in terms of his prime, and that prime in comparison to GK Harald, a topic which I'm sure would be immensely fun and not incredibly boring and pedantic to discuss, the important pieces to note are here:
-Harald has gotten more experience and stuff since he fought Rocks. He MAY be stronger than before even in base. Nothing indicates this, so I won't assume it.
-Harald is stronger as a GK than he is in his base form. This means he is stronger than Rocks here. I do not care. I do not care.
-When Harald and Loki first fight, Loki is using his non-Ragnir hammer, which is obviously weaker than Ragnir.
-Both use Conquerer's Haki, as seen by the big waves going everywhere.
-They clash directly.
-Loki notes that he has "monster strength" and is confused where it's coming from, indicating Harald at this point is using his GK-enhanced power and is not weakened in whatever manner people want to argue.
-With Jarul, he is able to overpower him.
-Loki is able to pass Ragnir's "test", which Harald indicates he'd be unable to do Pre-GK
This, to me, indicates that Loki is not inferior to the likes of Base Harald and Rocks, and is likely superior in terms of base physicals, Haki aside because idgaf.
Ragnir-Wise:
-I explained the test thing
-Loki's first hammer broke against it but not against Harald's sword
-Loki's able to kill a non-guarding Harald with a Hao Infusion + Lightning Bolt attack
This indicates that Loki's obviously better with Ragnir than without it, duh, why would you even add this SlendVeny?
Zoan-Wise:
-Harald refers to the fruit as having "legendary power"
-Harald gets his ass beat by Hybrid Loki with no Ragnir and gets ripped up without landing a hit. Loki literally smacks some sense into him
This indicates a high level of superiority. His Hybrid Form is stronger than GK Harald without using Ragnir at all.
Imo, being charitable towards the anti-Loki agenda, and assuming Loki hasn't gotten ANY stronger over time, we're looking at:
Hybrid Loki > Full Zoan Loki > Base Loki(Ragnir) > Base Loki(No Ragnir) =< GK Harald > Base Harald = Rocks
And what we KNOW for a FACT based on DIRECT COMBAT is that:
Hybrid Loki > GK Harald > Base Harald = Rocks
Now if you wanna say Kaidou > Harald or whatever be my guest but I can promise you I do not care in the slightest.