"Best" for Spielberg would be a fight between Raiders and Jaws, not ET (which is still great), in my opinion. ET is the highest grossing, but I think an argument could be made that Raiders has become his "most popular" film over time due to giving way to a series of massively successful films, a short-lived TV series and video games that aren't best remembered for filling up landfills.
In any case, I think Spielberg's relative high points are too close and his biggest successes all too pervasively ingrained in pop culture to be able to pull out one that could be easily agreed upon for both categories to qualify for this topic.
It's weird that "most popular" is, in some ways, just as difficult to define as "best." How about Richard Lester - which is his most popular movie, A Hard Day's Night or Superman II?