How old are you?
I love Witcher 3 and CD Project, I am their big fan and I consider Marcin Iwiński to be a very inspirational man, but I will never say it is the best game or best RPG ever. Because that’s just not the case and it’s not a matter of taste.
I played many awesome and more complex RPGs: Ultima, Gothic, Baldurs Gate, Fallout 1 2 New Vegas (not 3,4), SW: KOTOR, Morrowind, MnB, Dark Souls, Vampire: The Masquerade, Planescape Torment.
Even though Witcher 3 has a great script, amazing quests and characters. So compared to complex RPG lags behind in complex roleplaying and is closer to best action adventure than the best RPG.
Witcher 3 does not include more class, not many choices in dialogs like persuasion, stealth or more complex magic.
I consider Fallout New Vegas the best open world RPG because it contains complex roleplaying, great nelinear script, great companions, nonlinear quest, many choices in dialogs and many choices based on skill like persuasion, medicine, science, ect. ,stealth, and let me finish the game without a single killing or killing everyone (except children).
And from Kingdom Come I expect to contain everything I mentioned, plus adds cutscenes (I hope Mafia level and the dubbed main hero like Witcher 3. I think it can be the best complex modern RPG ever made (considering the absence of magic, other fantasy element and children).