Diner - never seen it
Bringing Up Baby - Major Hawks, major screw ball comedy, major Grant/Hepburn, so it's hard to argue with this one if you care about any of that stuff.
Yojimbo - I don't know what early, pre Kurosawa samurai movies were like, but from the era that he participated in an influenced this is THE quintessential samurai movie. Kurosawa is also really important because he brought an unusual amount of Western influence into his films that spread to other Japanese filmmakers (especially in the genre) but also had a big influence in the West because his movies were accessible.
Wild Strawberries - I haven't seen this in a while, but I thought it was one of the best Bergman movies, and if you only want one or a few of his/best most important movies I'd pick this one up unless there's something else you'd much rather have.
The Green Mile - I liked this when it came out but haven't seen this in forever. It's probably the least important movie on this list, so get it if it's a personal favorite but otherwise you can pass.
The 39 Steps - Early British Hitchcock has a different flavor than his later American movies, but in other ways he imitated this one fairly closely. So it's a must see if you care about Hitchcock. If only only want one of his early British era movies most people would take The Lady Vanishes and maybe that one really is better, but 39 Steps is important and alot of fun and makes a great companion piece to it's successors like North By Northwest.
Spider-Man 2 - One of the better movies of the early superhero era (don't ask me about the last 12 years), maybe the best, and one of the most popular. So if you care about that stuff get it. I don't think it's a movie that everybody needs to see though.
The General - I see people saying "Sherlock Jr. is better and funnier" and maybe it is funnier and more conceptually interesting but The General is the epitome of Keaton's combination of slapstick and death defying stunts.
Ace in the Hole (shout-out to BD) - Shamefully I have not seen this.