For me personally, it was (you guessed it) Joker. Ever since playing Persona 5 back in 2017 he was my #1 most wanted dream character for the next Smash Bros. game (Ultimate wasn't even announced at that point). I realized he wasn't very likely though given at that point all the 3rd party characters that had joined Smash were huge gaming icons (and Bayonetta but she was a special case due to winning the ballot and Nintendo kind of adopting her).
Then, a day before the release of Smash Ultimate at the Game Awards that trailer happened...boy it was perfection. I definitely lost my shit at that moment haha. Bonus points also to him being very fun to play (and winning Evo half a year later :P).
Runner-up would be Shulk. Similar situation here, as soon as I played Xenoblade back in like 2011 he became my most wanted character for the next Smash Bros. His announcement unfortunately had less of an impact though due to it being post ESRB leak and in a JP only Nintendo Direct so I didn't even catch the trailer live. Still, amazing nonetheless. Also, once again he's super fun to play especially in Ultimate.
Other runner ups include Mega Man and Hero who were my 2nd most wanted characters for Smash 4 and Ultimate, respectively. Pretty crazy that I got them as well.
Honorable mention has to go to Cloud fucking Strife though. That was definitely the most shocking, most impactful character announcement of the last decade on a general level. Watching that trailer I could feel the shockwave rippling through the internet. Like holy shit. Also pretty incredible to see that he became like one of the most played characters online and still is to this day.