Pretty self explanatory title. I glanced through the first half dozen or so pages on the forums here and I didn't see a topic dedicated to a favorite X-moment over the years. I'll start us off! My personal favorite moment doesn't involve an epic fight or some type of romantic scene. It's a return!
Now apparently this was the worst kept secret in comics at the time, but I was going to school and hadn't read comics for a few years. So around 2005/2006 ish I heard about this comic series that was flying off the shelves. I eventually bought into the series and around a half dozen issues in I was treated to my favorite character returning from the grave. I was beyond surprised/excited. Joss Whedon has given me much entertainment over the years and was solely responsible for bringing back Petey. He can do no wrong.