For me, the best E3 is 1999. Dreamcast was the new hardware everyone was talking about. SNK was putting out a new system. Sony showed off Omega Boost and for a certain somebody. Legend of Dragoon was at this E3. PS1 was at it's best. SEGA looked hopeful, SNK had something to offer and Nintendo had a solid E3. Capcom was outstanding. Everyone was amazing.

More photos of E3 1999 https://imgur.com/gallery/onDvupU

While I find E3 2015 to be pretty dull overall. My fave moment in any E3 despite how the end game turned out (it was awful) but for a moment 15 years of waiting Sony announced Shenmue III. My mind was blown.

I'm not sure what my favorite E3 is, but the biggest moment moment for me would be the E3 2003 Halo 2 demo. Sadly the final campaign didn't come close to the initial demo. 

Orchestra into live GoW 2018 demo was a pretty hype.

Favorite E3 is probably 2006 though I'd say 2010 was solid as well. Mostly speaking from Nintendo's perspective. Also fond of 2011, 2013, 2014, 2017 & 2018. 2019 & 2021 also weren't too bad, as well as 2005. 2001 had the iconic Melee debut.

As for favorite E3 moment, I'll let this video speak for itself: https://www.youtube.com/watch?v=VE2Dc1sx71U (by the way, it's really touching that the video starts with "Thank you, Mr. Iwata", even though it is entirely unintentional from his death)

It's a shame Nintendo stopped doing live press conferences for E3 because we missed so many potential freakout moments. Admittedly, the freakouts are what help make a moment the best in my opinion.

BTW, how do I get it to display the video in the post like other people are doing?

I have Jet Force Gemini and Power Stone. I don't have a fave E3. They all have good and bad moments.

Zelda: Twilight Princess announcement

I dunno about entire expos, but Sony's E3 2016 was perfectly paced and presented while their 2015 conference blew a bunch of minds with those FF7R and Shenmue 3 announcements.

Microsoft E3 2018: When they bought 5 studios. Hope to see more of that one day.

The only E3 "moment" I actually cared about since 2006. All the best stuff gets announced in individual company events these days.

Other than that, the PS1 reveal at E3 1995.

I was 6 years old in 1994 and 11 years old in 1999 and correlates to true peakness in those years fending off against those wars I know because I was old enough to go to school and lived it to defend hard mindfanboyism I got too many E3 will update later.