The two most moving, realistic moments were...
1) the son's reaction realizing his father was cracking up, and the father's reaction to this
2) the reunion scene at the train station
I first realized Spielberg was a genius with "Jaws" but not as a master of suspense, but humanity. I still feel one of the best scenes in cinema is the scene in "Jaws" is the impromptu visit with a bottle of wine to the Brody household when the sheriff got home after a trying day including the other best scene the widow's slap. But while that slap was shocking, the quiet intimacy of the gathering of three with a bottle of wine is for me at least, absolutely mesmerizing.
Back to "Close Encounters" - I also loved the chemistry between Richard Drefyus and Melinda Dillion - theirs was a true adult relationship showing the love that comes from mutual understanding, from bonding over shared, traumatic experiences.