Hard to say. Probably a Coen brothers film. My early short list would be:
O Brother, Where Art Thou?
Hail, Caesar!
The Big Lebowski
Not in any particular order, mind you, just that’s the short list. I might also add Indiana Jones and the Last Crusade and (perhaps as a dark horse candidate for the vote going on in my head) American Grafiti. George Lucas has done… questionable things, but he also did a lot of good.
A early Speilberg film: 1941. Written by Robert Zemeckis, and you see a lot of similarities to things he directed like Used Cars and Back to the Future.
So, Jackie Brown. I was close to going with Jackie, but Pulp Fiction ended up my choice. The problem with JB is that Rum Punch, the book JB is based on is just a bit better than the movie.
These comedies will never make “classic” status, but they ARE entertaining:
The Hallelujah Trail (1965) with Burt Lancaster and Lee Remick. If only for “Pilgrim” sitting out the battle in the sandstorm.
Support Your Local Sheriff (1969) and Support Your Local Gunfighter (1971) with James Garner. No more need be said.
Hot Stuff (1979) with Dom DeLuise, Jerry Reed, and Susanne Pleshette. A buddy cop story. Good chemistry between the actors.
Used Cars (1980) Kurt Russell. Kurt Russell is relentlessly cheerful in the face of adversity. He almost forces you to like him as he plans one scam after another.
A Walk in the Woods (2015) Robert Redford and Nick Nolte. Another buddy picture, with one dysfunctional buddy.
Animated; I haven’t seen the live-action version.
Great movies. Sheriff was my father’s favorite movie.
And the answer is: Smokey and the Bandit.
You can close the thread now. 
Errol Flynn’s Robin Hood and Captain Blood. They were a double feature at the local art house theater - the Majestic for you Madisonians - and I knew nothing about them except my dad was very excited to go see them. I was about 9 or 10.
“Lassie Come Home” when I was 10.
I think that’s the one I sobbed uncontrollably at.
Yes to all of these. But if I had to choose one, probably Guardians of the Galaxy. I just remember thinking during and afterwards: “Damn, if this isn’t an entertaining movie”. Not necessarily a great movie, but very very fun.
No horseshit, Jack.
I’m adding another nominee I haven’t seen mentioned: The Martian.
I know people have picked apart the science, but not being knowledgeable in that regard I was simply captivated from beginning to end. From script to pacing to visuals to acting, I thought it was close to perfectly executed, and the final scene utterly sticks the landing.
This is an excellent choice. My wife has absolutely no use for science fiction, and has seen this 3 times. (I like sci-fi and love the movie).
I’ll add The Sting, Jaws, The Godfather, Raiders of the Lost Ark, The Great Escape.
I’ll watch Major League any time, just for the last 2 innings of the climactic game. Ricky Vaughn coming out of the bullpen is one of the best scenes in any sports movie.
I’m glad and amused that Galaxy Quest is such a popular answer because it was a top pick for mine as well.
There’s also Moulin Rouge! which I saw when I was too young to be familiar with a lot of the songs (and a lot of the specifics of the plot, for that matter). But I loved musicals and that one was so wild and colorful, surreal, funny, and dramatic and felt really NEW next to, like, The Sound of Music and Oklahoma.
I saw Star Wars in its original run when I was about 15, so nothing will ever beat that for emotional impact and entertainment. It sure took a long time for the sequels to come out.
Other enjoyable movies were from Tarintino. Starting with Pulp Fiction. That movie really grabbed me. Right after the movie I got a piece of paper and wrote down the scenes in chronological order so I could better understand the move.
Both Kill Bills were wild rides.
Other movies that had a big impact were Crouching Tiger, Hidden Dragon and Wall -E
So many great movies already mentioned! 
For sheer entertainment value, I’ll add A Fish Called Wanda (I rarely LOL at movies, especially in public, but I laughed out loud in the theater – as did most of the audience) and Trading Places (the rare 2-hour movie where I don’t mind/notice the running time).
Star Wars, maybe, or Noises Off! or Galaxy Quest or The Sting…
I was surprised at the live action version. A lot of the same scenes seemed to have the same camera shots as the animated. Something I didn’t think they could do with live action. Like when Aladdin and Abu are being chased through the market for stealing early in the movie. That was very good. That actor had to be in very good shape.
I’ve watched several movies that left me in a daze after leaving the theater, a few after seeing them on TV. A shining example is The Thing (1982). I hadn’t seen anything like that before. Here’s some other ones that take my breath away even now, and give me all the feels (something I enjoy):
Ben Hur (1959)
The Exorcist
Evil Dead 2
Raiders of the Lost Ark
Aliens
The Joy Luck Club
Empire of the Sun
Groundhog Day
Avatar
Inception
Baahubali 1 & 2 (do yourself a favor and find these somewhere)
The latest movie to pull this off was surprisingly a Netflix original, Underground 6.