Nothing less than DVD these days, who wants to watch betamax/VHS/SVHS tapes anymore?
We have a few DVD's and a couple of Blu-Ray's. A huge collection of "digital content". HD makes a huge difference over older formats (as expected). The detail and far better sound 7.2 etc is worth it.
SD DVD stuff up scales well, depending on the original content. But watching HD movies is the way to go.
I even watch old stuff that has been released in HD format, which is great as you get a more movie like release as if you are watching it at a theater/cinema. Back in the day when movies were released to tape it was at a lot lower resolution than what you would get going out to the theater/cinema to watch it.
The end of this year/start of next year proves to be a very exciting time for movie buffs with the 4k and 8k standard becoming more mainstream.