I personally don't see how still unreleased games or games where the promises have a high potential to fall short can really be labeled objectively among the Greatest E3 moments of all time.
Otherwise it's voting for the best smoke and mirrors or false promises.
I mean, is the Duke Nukem Forever E3 2001 trailer one of the greatest moments in E3 history? It looked awesome at the time and generated a ton of hype....but we all know how it turned out in the end.
Watch Dogs E3 2012 demo looked awesome at the time and was out of left field....but once again, people aren't so quick to label its reveal as being so great given how the final product turned out in comparison.
I don't think I even need to go into how great the Aliens: Colonial Marines E3 demo looked vs. what we actually got.
For those being quick to list unreleased games as the greatest E3 moments....down the line, if that game ends up being a huge disappointment or is still unreleased several years from now, will you still claim its E3 reveal among the greatest of all time?
For me, the truly greatest E3 moments that are the ones that actually delivered on their promise, and where we can look back and point to the beginning of something great that influenced gaming and made it better.