Ah, I saw this first a couple days ago. Then a couple dozen more times since. Well done, Khoodos!
When I first saw the video embedded (on Cheezburger, I think), I thought:
"Hm, interesting. A song about the Pokédex. Could be fun."
*presses Play*
Then, the piano started playing.
"Wow. That melody... this WILL be fun."
And then it was revealed that you sing as well as you play the piano and write songs (And record). I've come across way too many examples of YouTube songs where one or more of these elements are sorely lacking, good as the rest of it may be.
Somewhere halfway through the first few lines, I was worried that it wasn't updated to Gen. VI. Then Bunnelby was mentioned, and all was fine.
The next few minutes were spent trying to find my jaw somewhere on the floor. It was awesome. Especially at the end, where I realized you even managed to end it with the franchise mascot. Though, I felt slightly disappointed that the three unreleased legendaries weren't mentioned.
Then came the coda, and all was good.
Now trying to learn it myself. It must have taken weeks to memorize, which is especially impressive when you consider that the names haven't been known for more than ~6 weeks (the hidden legendaries even less).
Also, don't mention the Megas. They don't have Pokédex entries of their own, which, given the song title, would mean they don't meet the criteria to be mentioned on their own.
Great job!