ONE(Japanese song)

ONE is the ending to Giratina and the Sky's Bouquet: Shaymin. I'm rather surprised this one wasn't posted here yet.
Here's the link with the lyrics> bulbapedia.bulbagarden.net/wiki/ONE_(Japanese_song)
And a video link if you wanted to listen to it> 

Now, if you've read the lyrics, I don't think this one needs much explaining... but here's an explanation anyways!
First for proof its talking about Dawn and Ash, one line reads "You gently give me the strength to move forward". This can be seen to apply since Ash helped Dawn through her depression, as well as that both Ash and Dawn support each other, always. Another is near the end, two of the lines reads "Thousands of flower petals come pouring down
 On us". This is relating to Shaymin, at the end of the movie, when a lot of Shaymin fly into the air, blowing petals everywhere.

As for the relation ship between Ash and Dawn, this is the chorus to the song:

You are the one

As an irreplaceable friend

And someone more than that

You gently give me the strength to move forward

For that, I love you

With a feeling that will never change

With this heart that never lies

I just want to tell you, "Thank you, my one"

That doesn't really need much explaining, so yeah, I'll leave it at that!