I still say ACO was easily the most impressive. I mean, 2001 and The Shining both look better, but you expect that with 2001 being Kubrick's prestige picture, and with The Shining being relatively new and having a massive fanbase. How good ACO's transfer turned out really impresses me, for being an older film/the bastard child of Kubrick's filmography.
As for the films themselves, 2001 remains at the top, but watching all these Kubricks again has vaulted ACO from the bottom tier to just a hair shy of 2001 for me.