As much as I love the original Blue and Red above all other Pokemon, I didn't really like Yellow all that much. Mostly because Pikachu was useless in the first gym.
So my choice is Emerald.
Side note: When Silver and Gold came out, I was one of those "original 150 ONLY" Pokemon fanboys. It was Emerald that opened my mind to the rest of the franchise (as well as making new friends that had continued where I stopped).
"Now bear my arctic blast!" ~Noon THEY NEED ROBOTREK ON VIRTUAL CONSOLE!