So, I only played 4 games from 2018, namely God of War, Monster Hunter World, Far Cry 5, and Red Dead Redemption 2.
FC5 is the worst out of the lot (for obvious reasons).
I enjoyed RDR2's main story quite a bit, though the side content was a mixed bag. The challenges are pretty bad (fuck Gambler challenge 8 ) and it certainly doesn't help that they aren't retroactive. Hunting is rarely worth the vast amount of work and preparation you have to do for it. Reloading my game multiple times so I can get a moose to appear, of which might not even have a perfect quality pelt when it spawns, just so I can craft a pair of purely cosmetic gloves is fucking ridiculous. Also, L2/LT being both the "aim" button and the "lock-on in order to talk to NPC" button caused so many problems for me and screwed up a lot of interactions with people.
GoW ranks a bit higher mainly because I was nowhere near as frustrated playing it as I was with RDR2. It had a few bumps along the way, though there were a couple of things that really stood out to me. First off, we have the way Muspelheim and Niflheim were handled. Rather than two entirely new areas with quests, loot, etc. that you could explore, you have a set of arena challenges and an randomly generated area that changes each time you enter it. To me, it feels like they were tacked on to the game to give the player more stuff to do simply for the heck of it. Secondly, we have the valkyries. I wasn't really a fan of the decision to make the player fight the same boss 9 times with minor alterations.
MHW is the best of the four. It's a game where you hunt monsters... and that's it. The game has a very simple vision, but what matters is that it realizes that vision. You might say that it REALLY MAKES YOU FEEL LIKE A MONSTER HUNTER.
...but yeah. I liked it. The main appeal for me is probably the weapon variety. Being able to choose from different 12 weapon types (longswords and dual blades are a myth and if you disagree then you should kill yourself, you weaboo piece of shit) that each have an entirely unique feel to them is radical. The only thing I hated about MHW was the arena... balancing the arena for people who play on their own was apparently too much work for Cap-ca-pom, so grinding for a set of armor with shitty internet is an absolute chore.
tl;dr - Monster Hunter World is my GOTY