Alien - I've always liked this one and its one of the movies on this list that I've rewatched the most. It hasn't really aged either. That said I'd be less inclined to try and persuade someone who really didn't like it to give it another chance than I would be with most of the rest of this stuff.
Mulholland Drive - I don't remember it too well but it is rightly considered one of Lynch's better movies. Usually this, Eraserhead, or Blue Velvet (which I didn't care for) is picked as his best. Or Twin Peaks but that's an entire series.
The Passion of Joan of Arc - I thought this was absorbing but I need to see it again to say much more about it. It's certainly a major classic though.
Sunset Boulevard - Maybe Billy Wilder's most celebrated movie and one of the greatest Hollywood movies about Hollywood.
Reservoir Dogs - It's easy to like this one even if you're sick of Tarantino.
Platoon - I'm only kind of on board with this one. The general depiction of Vietnam is potent and earns the movie an enduring reputation but I think the warring-father-figures device is too simpleminded and reductive.
Rebecca - I know I liked this although it's been a long time since I saw it. It's a classic but alot of people complain that the studio intruded too much and think his later movies were improvements.
Stagecoach - Of course I love this one. Nowadays it's not really fashionable to pick this as Ford's best since many of his later Westerns and other movies are more complex, but it's still beautifully crafted and terrifically entertaining. The Indian attack is a contender for Classic Hollywood's greatest action sequence. It was hugely important both technically and for the revival of the western as a respectable genre and not just b-movie fodder.
Bonnie and Clyde - Another one that I liked but haven't seen in a long time. It's one the early great New Hollywood movies, so you have to see it.