Hmmm... what were the open world games of 2020?
Assassin's Creed: Valhalla
Cyberpunk 2077
Ghost of Tsushima
Marvel's Spider-Man: Miles Morales
Watch Dogs: Legions
Yakuza 7: Like A Dragon
I'm sure/know there are more, but I don't feel as if open world was as prevalent this year? Some of the big games -- The Last of Us Part II, Final Fantasy VII Remake, Persona 5 Royal, Dreams, Ori, Demon's Souls, Sackboy, Astro, Wasteland, Godfall, Predator, Iron Man, Star Wars Squadrons -- weren't open world at all.
Haven't played Cyberpunk, but if we're talking just about the scope and tech, GTAV's world is impressive in plenty of ways but it was not my favorite open world unless I was in the city. I get what they did with the top 2/3s of the map, but I think that should have been scaled down and/or made denser if anything; it's not like RDR2 where there's plenty to stumble upon and discover and plenty of NPCs who know other NPCs (forgivable since this is old tech we're talking about for GTAV).
It's why I like Ghost of Tsushima because I love encountering the wildlife. I have fond memories on stumbling upon fox nests or heading toward smoke in the distance, which was better than just rural areas with nothing to do imo.
If GTA VI had a denser world that had more city stuff, with rural areas that have more to do, I think it'd be mind-blowing. But there weren't enough buildings to really explore or things to do in the sticks that would qualify it as the best.