I posted my comment about Bloodborne after about 60 hours with it. Now that I have spent over 80 (and counting) with Sekiro I will say this:
Full disclosure if not already clear: I like BB a lot and think it deserves GOTD.
Personally I like Sekiro more. Why? The combat is just ridiculous once it clicks, and the lack of RPG elements is a plus in my eyes. With BB I think they were looking for a way for the player to be more aggressive with their encounters, with Sekiro I think they got it right.
That said I think Bloodborne is the more universally appealing title due to the RPG elements. I could see why someone might be put off on Sekiro - Trouble with a boss? You're not grinding your way passed it, you have to figure it out after repeatedly getting your ass handed to you. Not everyone has the patience for that. That said I think the difficulty of Sekiro, like every other FS game, is overblown. It is challenging but fair.
Once you figure it out though you feel almost invincible. Well as invincible as you can feel in an FS game, but you know what I mean.
Bloodborne crushes Sekiro in the world/lore/atmosphere department, though. And Sekiro was no slouch in this regard. Bloodborne did Lovecraftian as good as Lovecraft himself.
GOTD is easily Dark Souls, in my opinion. I'm old enough to have been an adult with disposable income at the beginning and end of this decade, and I played all the hits, all the indie surprises, all the overlooked gems.
No other game left such an indelible mark on the medium. In twenty years from now, "Souls-like" will still be a commonly used descriptor for a video game, akin to "Metroidvania."
The Witcher 3 was fantastic and the high water mark for open world fantasy RPGs. It was sort of the culmination of all that specific genre had built upon. So it's close. But Dark Souls arguably saved mainstream gaming as a serious, challenging, artistic medium, and not some kind of derivative movie simulator (looking at you, Uncharted series).
If we're considering the wider industry implications of gaming, then League of Legends deserves a mention as GOTD for its role in taking e-sports to another level of popularity and professionalism. There are better games connected to e-sports (Dota 2, obviously), but none as big as League.