This is going to be a long review, so there's a quick summary at the bottom.
This game is incredible. The open world isn't too big that it overwhelms, and even if you have to ride for a really long time, there's always an encounter which keeps you on your toes. I wasn't bored playing this game at all. The map being split up into 3 different parts is genius, as it doesn't throw the player into this massive spit of land.
The combat is the meat of this game. Every kill you get is satisfying, and the mechanics are incredible. The perfect parries and dodges are satisfying to get, and every different enemy has their own style of fighting, so it isn't mindless button-mashing. You need to carefully think about how you approach every enemy, every base, and every encounter. At the same time, you still feel like a total badass. The combat is perfect.
The levelling and progression system is very good. It doesn't just give the player everything at the start, and you can sort of choose different categories to focus on. Improve your stealth? Ghost skills. Make combat easier and dominate on the battlefield? Samurai Skills. Want to discover more on the island? Guiding Wind Skills. Every time you get a valuable skill point, you have to think hard on what you want and what you want to progress.
The customization is pretty good. You get to choose what your sword sheathe and handle look like, and when it comes to outfits, you can choose from a hat/headband slot, a mask slot, and an armour slot. Armour gives you perks, but nothing else does. Head, mask and weapon customization simply makes you look cooler. There are so many different options, and you'll always find something for you. When exploring the island, you can find flowers. When trading with merchants, you can use flowers to either pay for a new headband/hat, spend flowers on a new sword colour, bow colour, or armour dyes. There aren't any cartoon-ish dyes in the game, all the dyes look cool. Customization is almost flawless. In my opinion, I wanted more armour kits, as there's 11. Keep in mind, there's a total of 41 headbands to collect. But this is one of the two things I found wrong with the game.
The story is written well. It's linear, so you can only go down one path, but overall the story is very good. I got attached to multiple characters emotionally, as Jin has been through so much with them. One thing about the story is there's lots of death, to the point of the amount of death being unnecessary. That, again, is just a personal opinion.
Overall, the travelling and open world is flawless, combat is super satisfying, customization is awesome, and the story is epic and game me chills. Hands down the best game of the year, if not the decade.