It's pretty debatable. As many have said, 2010, 2014, 2017, and 2019 are pretty close contenders.
2010 had Skyward Sword, Kirby's Epic Yarn, DKC Returns, Goldeneye 007 remake, Epic Mickey, Golden Sun: Dark Dawn, the unveiling of the 3DS, and the return of Kid Icarus
2014 had the epic Iwata vs. Reggie fight, the Robot Chicken skits, Zelda BotW's first tease, Splatoon, Smash 4's E3 coverage and invitational, Yoshi's Wooly World, Captain Toad Treasure Tracker, Kirby and the Rainbow Curse, Hyrule Warriors, and the tease of Super Mario Maker (I didn't mention ones like Bayo 2 and Xenoblade X due to them already announced from previous events)
2017 had the tease of Metroid Prime 4 (which may have lost some points due to the restarting of development), the tease of Pokemon Sw/Sh (debatable on how people are reacting to it now due to the recent events), Kirby Star Allies, Rocket League, BotW's first DLC pack, Yoshi's Crafted World, and the blowout of Super Mario Odyssey
2019 had the two Smash Bros. Ultimate DLC reveals, Luigi's Mansion 3's blowout, Trials of Mana Remake, No More Heroes 3, The Witcher 3: Complete Edition, Animal Crossing New Horizons, and Zelda BotW 2, among others
I probably have some recency bias, but I think 2019 was the best one out of the 4. A lot of games announced or showcase of new features/footage and some neat surprises like The Witcher 3 having all the DLC along with the game in one cartridge with no need for online updates just to install it, Banjo-Kazooie finally making their MCU Spider-Man-like return to Nintendo through Smash Bros. Ultimate, Trials of Mana getting a remake along with the Collection of Mana finally coming West during that day, Panzaar Dragoon remake, No More Heroes 3, and Zelda BotW 2.
I will give props to 2016 and 2018. I know some are not fond of the one-game focuses, but Zelda BotW set the tone for a new journey for Nintendo and the blowout at E3 2016 stole the spotlight even from Sony. I remember there was a video of people running inside the convention and apparently it was just to play BotW. The Nintendo feel with Nintendo's booth being basically the Great Plateau also added some unique charm that I think very few game companies can match in regards to standing out in a convention. 2018 was probably uneven, but Super Smash Bros. Ultimate, in the end, warranted the big spotlight. At 14.73 million units (as of June 30, 2019) and counting, Smash Bros. Ultimate is a huge system seller and was advertised as the ultimate celebration of Nintendo and video games with every fighter from the past Smash games coming back along with new fighters taking the stage as well.