The Empire Strikes Back (1980): My favorite film in my favorite film series, more or less. I don't really know if it defined the Darker and Edgier Sequel, but for me it'll always stick as the epitome of that particular trope. Plus, it has the best jokes in the series, too, and incontestably better execution.
A Shot in the Dark (1964): Best of the Sellers
Pink Panther films and probably my favorite comedy. This is the one that really defined all of the elements that made the series iconic, before they flanderized most of the characters. Clouseau was something of a bumbling fool, but he was a bumbling fool
who was right (albeit for the wrong reasons) and who did actual investigative work. Herbert Lom did a great job as Dreyfus for his first (and best) descent into lunacy. Has some great set-pieces (like the nudist colony) that don't degenerate into total onanism like some other Edwards comedy set-pieces. And Sellers' accent is actually semi-believable.
Casino Royale (2006): I love Bond films, and this one hit all of the high notes as far as I'm concerned. I mean...so much has been said in so many other places why this movie is as great as it is. Mostly because I'm lazy, I'll leave it at that.
The Good, the Bad, and the Ugly (1966): Some people call this movie the best of all time. Dunno about that. Classy, classy movie, though. Plus, it's entertaining to watch spaghetti Westerns and see
Spain the Old West populated by Southern Europeans.
Pulp Fiction (1994):
De rigueur Tarantino movie.
Also considered and sometimes making appearances in this list depending on when people ask me the question:
The Longest Yard (1974),
Jurassic Park (1993),
Gettysburg (1993),
O Brother Where Art Thou? (2001),
The Dark Knight (2008),
Jaws (1976),
Raiders of the Lost Ark (1981), and
WALL-E (2008).