It would be too easy to vote for T2 or Total Recall. [N.B. I'm shocked that no one has voted for either Running Man, and True Lies and Predator (!) got only one vote each.] Normally, I would have voted for Predator.
However, I voted for The Last Action Hero for one very basic reason: it is the most misunderstood, underrated action film (and AS film) ever, and Arnie actually does better acting in that film than in almost any other, especially when Slater steps into the "real" world. The film was so broadly maligned by the hyper-critical, anti-AS media that it missed the entire point of the film, which was essentially a brilliant deconstruction of the action film genre.
It saddens me that so many people don't "get" the film, and think of as somewhat "toy" or silly. It is anything but.
Peace.