What's your favourite game in the series and why?

I still haven't beaten Platinum, so I still can't say for sure, but G/S/C is still my top for obvious reasons.

Gold or FireRed. Gold has all of the fond childhood memories, and Firered was, well, kind of fun O_O. I haven't really played HeartGold yet.

Gold version, being the first one I played and all. Also, I could solo red in 5 minutes with my level 92 ho-oh. Sacred fire'd his entire team into dust.

Platinum being second, Giratina's battle music was awesome.

Loved Emerald.
It fixed some flaws in R/S and then introduced the Battle Frontier.
R/S/E also introduced a lot of cool stuff such as dive and double battle, and the graphics update compared to GSC was just amazing in my opinion. I also really liked the story, and the areas them selves.