Godfather and this was an easy call.
T2 is the greatest pure action movie ever, even though action movies of the 90's had considerably better plots than today's installments within the genre. It had everything you need for not just a great action flick, but a great movie in general. It had the biggest action movie icon ever, one of the greatest story tellings of the dangers of AI (a concept only iRobot told better imo), suspense that could pull any viewer in, and it was the movie that finally surpassed the Star Wars movies' special effects and raised the bar to a new level. But the special effects is really the greatest legacy this film left behind. Nothing else about this movie really stood out significantly from other movies, or even other Arnold movies, of it's time.
Lion King... okay... please, I like the movie and it is without question the greatest Disney movie ever, but I'm spitting in the faces of Francis Ford Copolla, Marlon Brando, Al Pacino, James Caan, and Diane Keaton by picking an animated film over the Godfather. You'd have to hold a gun to my head. No way.
Godfather is a movie that has yet to be surpassed in some ways. It's not a movie that begs for your attention like the other two. It's a very long and very powerful drama that gripped you in ways that was unheard of in the early 70's. It demonstrated often frightening aspects of an organized crime syndicate while simultaneously preaching strong family and social values. It did this without even trying hard, which is truly why the movie was so phenomenal. And it couldn't have worked without it's carefully-selected cast or it's legendary crew. Godfather is just one of those movies that will still be some people's favorite movie in 2072 and I really doubt Lion King and T2 will be.