Possibly Blue and Gold.
Blue was the first Pokemon game I ever played, so already it has the strongest nostalgia factor going for it. I did play Yellow, but I don't remember being quite as wowed by it. Plus I'd rather have another starter other than Pikachu, stupid first gym.
Gold, same basic deal as above except instead of being the first of the games I ever played, it was the first of the second gen games. I tried Silver, but I couldn't get into it as much - possibly because Gold was "mine" while Silver "wasn't mine" (might have been my sister's, I don't remember). Plus I think something silly like Gold having Spinarak over Ledyba was also the reason I liked it more, ah ha.
I might have liked Crystal better, but I sadly never got a chance to ever play it when it was current.