They're all equal, the reason Diavolo and the others went towards Polnareff's arrow so desperately was due to it being the nearest arrow around: the one who'd get it would get Requiem. The main issue is the ambiguity of Requiems and all that.You can even notice in the dialogues how they always talk about "the arrow": never mention it at "that arrow" or "requiem arrow". Diavolo even says "the arrow does more than just draw out Stand abilities?" refering to all arrows, never saying "he has a special arrow" or something like that.
To give an example I'd go as far as saying Bites The Dust is some sort of evolved stand in the same vein, there are several claims against that:
- Bites The Dust didn't make KQ sentient or it didn't change form, that's irrelevant since not every stand evolves in the same way (Chariot Requiem didn't ever say a word unlike GER, not even durignt that time Polnareff unlocked him while alive; some stands get Acts when they evolve, others like Star Platinum: The World stay the same),
- The arrow stabbed Kira, not the stand... yet the stand was in the exact same place than Kira (it was probably targetting Killer Queen instead of Kira)
- It's not named Requiem... yet Kira had no idea about such type of naming (something Polnareff came up with)
And most importantly, in all cases it was a really powerful evolution that answered the main desperate need of its user (Polnareff wanting to get some advantage against Diavolo - putting his stand against himself is a good way to do so - and eventually consolidated his desire: to protect the arrow; Kira wanting to be able to keep himself unnoticed; Giorno wanting to defeat Diavolo). One may say why didn't the arrow tried to stab GE or KC like with Kira... I'd say with so many stand users around the arrow wasn't driven to do so (also Kira was way more desperated than any other Requiem user).
Giorno not having a Requiem after being hurt by Black Sabbath doesn't mean a thing, Diavolo's KC was later harmed by the "Requiem arrow" and he got nothing. Requiem is not Super Saiyan, or a shikai/bankai; it's not an ability every stand user can develop and it's not inherently related to the original abilites of the stand. It's extremely circumstancial, as some sort of rework of the stand to allow it to defeat the unsurmonable odds it has to face. I'd say that happened for a series of reasons:
- Giorno never considerated he was 100% unable to best Black Sabbath unlike Diavolo, he thought he could defeat him by using GE's abilities in a clever way. On the other hand he considered he had zero chance to best Diavolo with base Golden Experience.
- Same with Diavolo, what could the arrow offer to someone who consider he had the best stand in the world? If it weren't by Diavolo having to share a body with Trish and the presence of SCR he could just wreck all of them in a pair of minutes and be done for. He was never desperate enough to consider he could lose until Giorno unlocked his Requiem
I had even read people saying the difference is that the "Requiem arrow" allows both the user and its family to get a stand (like DIO and Pucci did, when Koichi's family got nothing). Yet that doesn't mean every single person can get a stand or would get a self-killing stand like Holy or Masazo Kinoto. The old man Black Sabbath stabbed with the arrow died instantly. Also there are too few instances of people with families being stabbed with the arrow (DIO, Pucci, Koichi.. as far as I recall that's all) and several of them are sort of unknown (we don't know how the Nijimura family got their stands)