Definitely a huge delayed response on my part, but it's my GOTY for sure. Beats out Zelda: Breath of the Wild for me.
One of the best free roamers ever, with great, fluid combat. The story is amazing as well, and Aloy is an excellent protagonist to add as one of the faces of Playstation.
I definitely had some open-world exhaustion for awhile there though, considering Horizon and Zelda came out within a few days of each other, so it was hard going back and forth. Ultimately, I put down Zelda for awhile so I could keep playing Horizon, which I think says a lot of which game I like better. THey're both amazing games, of course, but I think it depends on taste. Horizon is more of a tribal, sci-fi, open world, robot animal killing game with great combat. Just the things you can do with the bow, while running around, setting traps, etc. It's really good.
Zelda is obviously good too, and I'm still trying to play and beat that too, but I think considering I finished Horizon's main story and side quests long ago, and almost Platinum'd it says which game I like better. The Horizon DLC is something else too. Some of the best snow physics engine ever.