Technically? Flight Sim, and it's not even close. It looks incredible on X|S, but my GOD does it literally look photorealistic on PC, and the scale of being able to fly everywhere on planet Earth at any given moment? That's something that no other game will be able to match this gen, mark my words.
In my personal experience on PC, in no particular order: Flight Sim, GotG, FH5, Fenix Rising, Control, and RDR2. Hell, toss Doom Eternal in there as well.
For Series X specifically: FH5, quality mode. Whilst I haven't seen this version of the game in person (PC), I've seen enough videos to know that this game is an absolute showpiece for the Series consoles, but especially for the Series X.
And, for PS5: R&C: Rift Apart. Technically, what the game is doing is amazing with the SSD, but visually as well it looks damn great.