I think good for their time films are things that were influential and popular at the time, but now just aren't as enjoyable. Films like Casablanca and Gone With the Wind will always be timelessly enjoyable. What always springs to mind for me is Citizen Kane, influential yes, but it is just not enjoyable at all and I would definitely not watch it again. I'm always disappointed when it tops "best of all time" lists, or if modern film makers cite it as an influence. It was influential for years afterwards yes, but still? It's like how every single band in the world say they were influenced by The Beatles just for the sake of it.
Things like Psycho or And Then There Were None as well I think, would have been scary at the time, now they're more like an unintentional comedy.