Movies like Jurassic Park, Titanic, and the Potter series are certainly epic in scale (both thematically and technically), and there's a lot to be said for grand ambitions. God knows the public ate them up upon release, and they've since become part of our collective pop cultural experience.
But are they cinematic "masterpieces?" The first two will probably achieve "classic" status (if they haven't already), but I would argue their shortcomings are significant enough to exclude them from the "masterpiece" list. For all it's box office success, Jurassic Park is in some respects a pretty sloppy movie. And I don't care what anyone says, the dialogue in Titanic is laughably awful.
That doesn't mean they're not big, fun, memorable movies, but from an artistic standpoint I think both Spielberg and Cameron have done better work (especially Spielberg).
As for Potter, I think the novels constitute a sort of literary masterpiece, but the films feel pretty uneven to me. And, frankly, a bit soulless. Just one guy's opinion, of course.
It's funny, the more I ponder the question of this thread, the more surprised I am by those titles I'd qualify as 21st century masterpieces. I knew, for example, that I liked and admired Cameron Crowe's Almost Famous and David Cronenberg's A History of Violence, but it never occurred to me that I regarded them both as masterpieces. Thinking about it though, I find they fall into that rare (and admittedly subjective) "perfect" category... two very personal films by gifted filmmakers that I personally cannot find fault with. I mean, there is NOTHING I would change about either of them, nor can I imagine anyone else having executed them so beautifully. As far as I'm concerned they're unique (and refreshingly unpretentious) works of art, which I suppose is as good a definition of "masterpiece" as any.