The "Save Marbule" sidequest definitely counts as well.
Discovering the Frozen Flame in Chronopolis and speaking with Robo is pretty memorable, too — not "awesome" as in "cool", per se, but the imagery, music, and information being conveyed to you at that moment are all pretty awe-inspiring.
Successfully casting the Chrono Cross Element in the final battle with the Time Devourer.
