These are the nominations so far for the most memorable anime episode:
Angel Beats #1
Aria the Origination #9
Azumanga Daioh #10
Banner of the Stars #1
Dennou Coil #16
Divergence Eve: Misaki Chronicles #13
Haibane Renmei #1
Higurashi no Naku Koro ni #4
Kamichu #3
Katanagatari #4
Mahou Shoujo Madoka Magica #10
Mushi-shi #1
Mushi-shi #5
Mushi-shi #15
Mushi-shi #16
Mushi-shi #17
Mushi-shi #22
Naruto #22
Paranoia Agent #8
RahXephon #18
Serial Experiments Lain #13
Sugar, a Little Snow Fairy #24
Super Dimension Fortress Macross/Robotech #18
Tengen Toppa Gurren-Lagann #27
Some other episodes were mentioned, though not actually nominated:
Angel Beats #5
Azumanga Daioh #18
Kamichu #5, #9 (“Yamato”), #12
Kino’s Journey #2, #3, #13
Surely there’s more. Where are Cowboy Bebop, Cardcaptor Sakura, Angelic Layer, Princess Tutu, Nadesico, Suzumiya Haruhi, Noir, Petite Princess Yucie, … ? Please make suggestions in the comments. Feel free to explain your choices in general terms, but be careful of spoilers.
When I post the poll, I’ll probably limit the number of candidate episodes from a particular series to two. For Mushishi, those will mostly likely be #5 and #16.
Research aids: Wham Episode; Moment of Awesome.