Wait what? A dark wraith that spreads unending nightmares shouldn't be legendary? A Pokemon that makes a barren wasteland burst into eternal bloom can't be legendary? A Pokemon who's rarely seen and banishes the powers of darkness doesn't count? What about an eldritch abomination that sits on top of the world you can summon via flute?
I get you don't like the event legendaries much in Gen IV, and Heatran I guess is borderline because aside from it guarding a rock in a volcano it's just kinda there, but you can't just discount a Pokemon from being legendary just because you don't like it. If it does something or has a quality that inspires some kind of folklore then it should be legendary; Darkrai causes unending nightmares for everyone within close proximity, so it qualifies as legendary. Shaymin can transform a barren wasteland into a fertile valley through gratitude; it qualifies also. There were a ton of legendary Pokemon introduced in Gen IV and Gen V, but unless they don't have any lore attached (Meloetta) or are just kinda there with no explanation on why they're legendary (Genesect) then they should remain legendary. The Kami trio causes severe weather wherever they appear, and even feuded for a bit before their brother showed up and got them to calm down, so you can't really say they they aren't legendary in their own right. That's what a legendary being is; it's either extremely notable itself or does something notable and rare and that quality causes people to spread stories and legends about it. Manaphy has the power to bond with any Pokemon and is extremely rare to begin with, and that alone is enough to make it legendary, though its offspring Phione is sorta debatable. As for the sheer amount of legendary Pokemon, well the event Pokemon aren't really justifiable at all but there are tons and tons of legendary creatures in the real world, and a lot of legendary Pokemon like the Kami Trio, Ho-Oh and others are based on those real life myths. There's so many cryptids and mythical creatures out there it's bonkers.
As for Diancie, I think it deserves the title. It is the queen of the carbink after all, and it's not only awe-inspiringly beautiful but is made of and creates diamonds, which could make anybody who has one rich beyond their wildest dreams assuming the diamonds don't just vanish after they're summoned.
