Mine would have to be Ruby
Ruby was the first Pokemon game I played. After awhile I got Sapphire, and I got every other game after that leading up to Platinum (never got any new pokemon games after that) and heck I even got Crystal.
But Ruby is my favorite. (Besides Deoxys, which you can't really get legit) my favorite legendary is Groudon. Mudkip is my favorite starter. I love Jirachi, Rayquaza, and Kyorge is pretty cool, but I personally like Groudon better. I just re-bought Ruby and i'm loving it so far! Now I could have bought Emerald to get the feel of both Ruby and Sapphire. But Ruby will always remain my favorite. I got my first lv. 100 in that game, and that was the first pokemon I beat.
I'm not planning on getting HeartGold/SoulSilver or Black/White or whatever. But if they remake Ruby & Sapphire for the 3DS (possibly DS) then I am for-sure buying the Ruby remake!