Seconding Blade Runner 2049.
End Game was a very underwhelming transfer, almost criminally so given the context. My favorite MCU 4K from an image quality POV has been Thor: Ragnarok, but I hear the Guardians of the Galaxy 4Ks are really good.
Adding Mad Max: Fury Road, still remains the most impressed I've been by any 4K image since upgrading my TV last year, and I've only got a lowly TCL 65R617. With your OLED, it should be absolutely smashing.
But yeah, I had the same reaction to 1080p BRs and how good they looked.
Also, off topic but just covering off, make sure to check out Planet Earth II (disc or Netflix). The image on Netflix had me drooling so I imagine the disc is bonkers.