I watched Guess Who's Coming to Dinner years ago (probably about 6 or 7 years now) and I had such a pleasant reaction that it's still carried with me. Plus I have an abnormal appreciation for Spencer Tracy (I mean I watched a good 20 films just because he was in them, and my favorites from other decades will reflect that). Plus there's something truly somber and beautiful watching him in his last film, I have a similar melancholic apperciation for John Huston's The Misfits (which was one of my bubbling under films). Both Gable and Monroe were outstanding in that film, and perhaps never better (although I certainly think San Francisco is a close contender for Gable, ironically also with Spencer Tracy).
This list ignores some of the better acting jobs of the decade, and I think there's only one Richard Burton film listed. However I think his turn in Huston's Night of the Iguana is exceptional, and despite the overly theatrical overtones of it Becket was certainly one of the best acted films of the decade. However as we've already mentioned, keeping the list to the essentials is necessary, because sometimes the overly long lists get a little distracting. Regarding the last two decades though, the films listed are generally easier to come across, whereas the further back you go, the harder it is sometimes to find these films, so for people using these as suggestions for future viewing it helps to keep it small here.