Jurassic Park without a doubt. I know people hate on The Lost World and JP3, but I love them both, albeit way less than the first movie. I don't consider Jurassic World to be a part of this franchise even though it technically is.
Second place would Indiana Jones with the exception of Crystal Skull. Raiders and Last Crusade are by far the best although I've always had a soft spot for Temple of Doom.
A fairly distant third is Back to the Future. I don't dislike these movies, quite the opposite actually, but for whatever reason they never had the same impact on me they did for many others. Funny enough my favorite is actually Part 3, followed by 2. I've only ever watched the first movie a few times, but I just never enjoyed it as much as its sequels.