I also find it quite funny how people list other reveals like Lucario, but at the same time forget that we had Ho-Oh in episode 1 of the anime
I just think back on those times as the franchise not knowing what the heck they stumbled into.
Like you walk out your door and your entire front lawn became a gold mine. You kinda just go, "Uh, um. Uh. Well. I guess I can do... uh, this? And, uh. This?"
Or in the franchise's point of view:
A: "Uh, so we'll throw this Pokemon in here, okay?"
B: "Why? What is it?"
A: "I dunno. It's cool? Maybe?"
B: "'Kay, see what happens? Right?"
A: "Yeah."
B: "Hmm. Okay."
Two years later...
A: "Remember that bird thing we had in Episode 1?"
B: "What bird thing? ...Oh, uh, that bird thing. Yeah."
A: "Let's make it an actual thing?"
B: "'Kay."