Man, I remember back when Snake was revealed. I wasn't even a fan of Metal Gear at the time, but even I knew this was a huge deal. (Actually, Snake being in Smash is what got me into Metal Gear, so thanks, Brawl! XD ) That one reveal opened up one hell of a can of worms. Suddenly, the impossible felt possible for Smash and that our wildest character dreams were actually within the realm of possibility. It was an incredibly exciting time to be a Smash fan, and I remember the build up towards Brawl's release as one of my highest peaks of hype as a gamer.
Then we got Sonic the Hedgehog. I grew up on Sonic, so I nearly lost my little teenage mind when I saw that he'd been confirmed. The Nintendo forum I was on at the time erupted with unanimous glee. It almost felt too good to be true, as we were all used to those infamous April fools screenshots and unlocking methods for Sonic in Melee. Man, Brawl's hype cycle was certainly a thing to behold. XD
The only other reveal that's gotten me as close as my hype for Sonic was actually Simon, as I love Castlevania, and have wanted him in Smash ever since third parties were revealed to be a thing.
TL;DR: My most hyped newcomer is Sonic, but Snake's reveal certainly got me incredibly excited for the possibilities of third party fighters in Smash.