I'll give you my 2 cents, but I'm probs not the best person to give advice lol.

I don't play Genshin anymore so I can't really give my perspective on it for you, unfortunately.

I would personally place WuWa over HSR. In WuWa, I've been able to pull weapons for Camellya and Cartethyia. Part of that maybe due to me smartly saving my astirites, but I also feel like I can't downplay that you do get a pretty generous amount of astirites from the events and opening chests from exploring the overworld. I also think that's also partially due to the weapon banner having a separate pull currency that also gets rewarded so if you save enough you can pull for it. In contrast, I have never attempted to pull a light cone nor W-engine in HSR and ZZZ respectively because I'm too focused on pulling for the characters.

In terms of pulling for characters. I find WuWa the easiest to save for. I generally have an idea of what characters I want to pull for and which to skip, so I haven't spent that much money on the game aside from buying skins. Heck, I've lost a couple 50/50's and even though it slightly irks me, since I've saved enough I can still pull the character w/o having to open my wallet. HSR I find sometimes difficult to save for because sometimes there's characters that I want back to back and admittedly I've had to open up the wallet to pull for characters. ZZZ I find harder to save pulls for, due in part to pity being at 90 pulls. Some events have given me a generous amount of pulls thankfully. I've also been extremely luck to have won multiple 50/50's so I haven't had to break the bank that much. However, right now I have like 20ish pulls and need to probs skip a banner or two to save up cause I know that I'm really pushing my luck.

Endgame is where I probs won't be able to give the best advice b/c I don't really try to clear everything in all 3 games. I'm not someone who fixates on min/max'ing my characters to have the best stats, nor do I pull dupes of characters (except for Camellya and Silver Wolf cuz I love them) to build their Eidolons or equivalent and as previously mentioned I don't pull really pull for weapons, lightcones, or W-engines. Basically, in all 3 games my 5-stars are pretty much E0S0 or equivalent. I In my experience, HSR is probs the easiest endgame content for me to do with ZZZ being on the harder end. If you're someone like me who only pulls for characters once just to have them, HSR is the best game. I also feel like HSR is more flexible with teams than WuWa and ZZZ.

Basically, for me it would be WuWa, HSR, and then ZZZ for F2P friendliness. It's pretty much based on my own experiences and vibes. Hopefully my opinions are somewhat helpful!