It is the only movie from my childhood that I think more highly of as an adult. Most are fun because they are fun, but The Princess Bride has the following going for it:

Every scene drives the movie forward and also gives insight into the characters at the same time.

The lines stand on their own even outside the context of the movie as it is just well written as a whole.

The editing has excellent timing for a lot of the situational humor.

The performances are solid, Andre the Giant is probably the least capable for acting/line but it fits his character and he has charisma so he gets a pass.

The love story is a self aware love story and the contrast between the story being read to a child and the story events makes it a fun version of a fantasy love at first sight cliche without needing to subvert expectations.

Quicksand!

It really does hold up as a well made movie that doesn't have filler scenes or a meandering plot and the third act really pays off on the prior parts. It isn't a masterpiece of film making that broke new ground and should be watched just for the experience. It is a very competent and well done piece that is fun to watch.

That said, my wife hates it so it isn't for everyone.