I'm still playing through the series, I finished 0 and Kiwami, still waiting for my gpu so I can play K2 (it uses dragon engine so I need something that is mid-range by today's standards)
For the two games I've played so far, 0 is my fav, Kiwami feels a little worse boss wise and fighting wise, EXP system isn't really my thing, I preferred the Money for upgrades system over it
I see, you're gonna love the rest of it. I played 0, 1, 2 on a PS2 emulator, skipped 3 due to the combat simply being unfun but watched it on YouTube, then 4 is really quite something. 5 is.. idk. It's good but it's weird. 6 is wonderful too and 7.. never played due to the combat.
I started with 0 and then played the others, and I have to be honest. 4 and 6 were kind of weird for me. I didn't really get into them because 6 was too short, and I found the map smaller. I have no idea if it's the Dragon engine. I just need to play the games again, and then I might write an update here.