That was what I came in here to ask, which further begs the question: are we basing our answer SOLELY on movies that Spielberg directed or upon any movie where he was involved in any capacity?
Poltergeist was directed by the great Tobe Hooper of Texas Chainsaw Massacre fame.
ETA: going through the list I realized all of the movies were directed by Spielberg, but you’d be amazed how many people think he directed Poltergeist.
That includes IMDb Trivia:
Is it possible to direct a feature length film when no one is looking? 
Tough, tough, tough. I ultimately went with Jaws, but wanted to actually go with Catch Me If You Can an exceptionally well made and watchable movie.
I’ve read interviews with Rubenstein saying that Spielberg directed the movie. I even started a thread about this Very Important Issue: Poltergeist directed by Spielberg and other possible "miscredits". - Cafe Society - Straight Dope Message Board
I would say Raiders (then Jaws, then Jurassic Park) is my favorite of his films, and Schindler’s List is (of course) his best film, to the point that he kind of undermined himself for Saving Private Ryan; people thought, “He’s already done his WWII movie with Schindler’s List.” Besides, aside from that opening sequence, there’s not much to Saving Private Ryan.
I’ve always liked Empire of the Sun (I’m a sucker for the “child in danger” trope anyway), but that has more to do with Christan Bale than Spielberg.
And E.T. and the other Indiana Jones movies are just garbage, especially Temple of Doom.
I went with Minority Report. Close tie with Raiders though.
For a LEAST favorite one, I am going to say 1941, with Kingdom of the Crystal Skull a close second.
I never understood the “Kingdom of the Crystal Skull” hate. Even with the refrigerator scene, and the aliens at the end.
The matinee serials and pulp fiction that Indiana Jones was based on were all pretty corny, cliched, and over-the-top. With “Kingdom of the Crystal Skull” Spielberg took all these pulp elements, stuck them in a blender, and turned the blender on high. He then combined the results with his always-excellent-if-not-rather-formulaic direction and produced a real hum-dinger of a matinee film.
(Spielberg’s success with “Kingdom of the Crystal Skull” is in direct opposition to Lucas’ failure with the Star Wars prequels. Lucas kept all the bad directing, acting, pacing, and script-writing of the serials, added an even more incomprehensible and Byzantine plot, and wrapped it all up in too much CG.)
Hard to pick **one **- but Raiders.
I’m normally just a reader of these boards but I couldn’t resist jumping in here. Hi!
I couldn’t choose just one film. I love Raiders, Last Crusade and Jaws. Does anyone else remember the tv movie he directed called Something Evil? It starred Darren McGavin and Sandy Dennis and it really freaked me out as a child.
Raiders was an example of the matinee format done to absolute perfection. Skulls is an example of its creator just taking us for fools.
The thing that pissed me off the most about Crystal Skull was the blatant reveal of the Ark of the Covenant. The last time we saw it, it was being buried in a mountain of bureaucracy, never to be seen again. But wait! Not only is it sitting out in the open at the end of an aisle, but the chase scene manages to bust it open. Certainly, the next day the janitor is going to come by, have to pick up the pieces, refile everything, and might take a peak at this amazing huge chunk of gold sitting in front of him. Completely ruins the ending of Raiders.
What Spielberg should have done is kept the Ark buried in a wall of crates, and once the chase scene zooms by, linger on the box with the swastika on it. A subtle wink that everyone would have picked up on, without being beaten over the head.
Raiders by a mile, although Close Encounters, Minority Report and Saving Private Ryan are all contenders. Never saw Schindler’s List, although I know I should. Someday…
Should’ve added: The Terminal and Catch Me If You Can would be right up there, too.
A few of the classics are films I don’t particularly like, including Jaws and Close Encounters. But my favourites are all the Indiana Jones films and ET, though I also like a couple of the oft dismissed ones like War of the Worlds and The Terminal.
I picked Last Crusade.
Raiders of the Lost Ark. I went to see it 18 times in the theater. Never done that before or since.
‘It ain’t the years, honey. It’s the mileage.’
I prefer early Spielberg (ranking Raiders #1, Close Enc #2, Jaws #3) where he used slow methodical suspense and expansive composed shots. His later movies have fallen in love with close-frame shaky-cam action and have lost what made his early work special.