Exactly as the title states. What pieces of mythology and lore behind the Legendary Pokemon are your personal favourite.
Personally, I tend to prefer the story behind the Legendary Beasts and Ho-oh in Johto though Giratina's backstory is a very close contender.. Crystal was my first Pokemon game so it has a bit of a nostalgia factor to it, Ecruteak itself is my favourite town in the series after all, but I always liked how mysterious they are, there's barely anything you see of them aside from the initial encounter at the base of the Burned Tower outside of wild encounters you have with them. Entei the fierce, Suicune the graceful and Raikou....Honestly, I feel like Raikou kinda gets shafted. They show how fierce and dominant Entei is through it's depictions while Suicune is portrayed as graceful and wise. Meanwhile Raikou is essentially a blank slate. Interesting thing is how their types line up with what actually occurred. Raikou represents the lightning that struck the tower, Entei represents the fire that engulfed it and Suicune represents the rain that put out the fire. While what they were before the tower burnt down will most likely remain vague for eternity, their legend lives on.
And then comes what is essentially the equivalent to the devil in Pokemon. Giratina. A Pokemon that once helped shape the universe, one that governs over anti-matter and was later banished for it's violence. It is now said to gaze into our world from another realm; one where neither time flows nor does space expand. It's said to appear in ancient cemeteries. One can notice straight away that there's a certain number pattern going on between it's 2 forms. 6 wing spikes, 6 ribs, 6 legs in the altered form and 6 wings, 6 leg-spikes, 6 ribs in the Origin form. While it's never been confirmed, Giratina is pretty clearly implied to be the 'Devil' Pokemon, even it's species name indicates this even further as it's appropriately titled as the 'Renegade' Pokemon. What's even more interesting is that it's creator and trio master, is the equivalent to god in the Pokemon universe. The distortion world is a very dark, dreary and maddening place. It's intriguing how despite the fact that it's never actually stated that Giratina is the 'Devil' Pokemon, the symbolism and lore behind it all indicate towards that being true. There's simply no way this wasn't an intentional design choice. Perhaps they'll touch upon that if they ever remake the 4th generation games.