That and Gen 2. There weren't too many legendaries in respect to the other Pokemon in those gens. Plus those legendaries did not serve too much of a focal role in the games (they weren't total gods or something like gen 4, just extremely rare) nor did they have too much backstory in them; they were assumed as myths by the majority populace, leaving a sense of mystery to those legendaries, and you as the player had to go out of your way to find them to prove if it is real or not. That scene in the first episode where Ash saw that Ho-oh not revealing what it was made it even more captivating and allowed us to wonder so many questions.

Though I still loved Mega Rayquaza and spamming Dragon Dance + Dragon Ascent back in ORAS. I also love Dialga and Deoxys.