ef: A Tale Of Memories has its share of very sad moments, in particular anything involving Chihiro Shindou's Ill Girl condition. Especially heart-breaking is the scene where Chihiro rips out the pages of her diary in which she mentions Renji, so she will forget him.
One particular tearjerker was the scene where Chihiro said "I can't go thirteen hours without thinking about you!" When Renji first finds Chihiro before running off to find the pages, and she claims doesn't recognize him, viewers know she's acting and and makes them think about how impossibly difficult and heart-breaking for her to be doing that...
The scene in which Miyako fills up Hiro's cell phone with increasingly desperate messages after he stood her up on their date also qualifies — especially since all we see is her standing still and her messages written out on the screen. Pretty intense. Kudos to the voice actors, too.
A tearjerker on its own, but twins especially will probably be hit hard during the flashback of the accident that caused Chihiro's amnesia. Seeing poor little Kei crying in the hospital and obviously believing it was all her fault made them cry more than the two above mentioned scenes.
And for the second season, Yuuko's backstory in episode 6 (crossed over with Mind Screw) and Yuu's and Yuuko's meeting in the final episode. God dammit Yuuko, why didn't you Look Both Ways?
Speaking of not looking both ways, when Yuu finds Yuuko's body and then picks her up, carrying it with Miki? Yeah, that's bad. Then Ebullient Future begins to play. Many bawled.
Yuu's and Yuuko's final farewells in the last episode. Some were a little confused as to why the camera kept focusing on Yuu's eyes while he was talking about how he had moved on and would be fine without her. Then a line from episode 6 comes up.
Yuuko: Yuu-kun, did you know that you blink a lot when you're lying?