What I've included here is the original, and the "remake" of sorts. Although both have the same premise, they are vastly different.
Both are brilliant demonstrations of minimalistic film-making, and are set in one single location: A big, deadly cube.
While the 1969 version directed by legendary puppet-master Jim Henson is a rather clever dive into metaphorical psychology that hits the nail on the head time and time again, the remake directed by up-and-comer Vincenzo Natali, is a simply entertaining thriller about seeing people getting killed off in fun and creative ways and wondering who will be left at the end.
Both feature somewhat flat acting at times, and constant dialogues about highly advanced theories about either psychology or mathematical equations might go over most people's heads, but that doesn't lessen the experience of either film in the least.
For this is actually a rare occurrence of both the original and the remake being very good in their own ways.
Don't bother with the recent sequels though. They're a waste of time.