It's the 19th-best game of the generation by my estimation, but since I really liked this generation a ton, it being one of the 20 best games is still quite the accomplishment. Therefore, I tip my hat to The Witcher 3: Wild Hunt, a more-deserving winner than most. (albeit, somewhat grudgingly)
The 2nd, 3rd and 4th-placed games really are among the top 5 of the generation, though, so GAF mostly knows its stuff.
Well considering the actual meaning of GOTG (a game/s that influenced games through out the generation) The Witcher 3 deserves it, also Souls games and PUBG.
Objectively speaking games of the generation aren't necessarily beloved by hardcore fans but games that create a trend and as much BOTW is a very good game it didn't influence any games yet to be considered one, nor did MGSV and a number of fan favorites in the pool, that's why i said from the beginning that this is a popularity contest not an actual look into games that left their mark through out 2013-2020.
No Odyssey is influenced by it's direct prequal Origins and even that's debatable since they're both (and Valhalla) influenced by The Witcher 3 and have been in development for years (at least 5 according to the Ubisoft GDC talk in 2016/17).
As for Immortals the greek game you mentioned it's also debatable since it's made from Odyssey's skeleton according to the game's director who's also directed Odyssey's, and nothing so far besides the art style is remotely BOTWesq but we will see.