Any answer other than Crystal wouldn't be very realistic, considering it is the best game. It pretty much took Gold and Silver, polished up what needed polishing, added stuff to enhance the game (see Blade's post :p), and is generally just a better game than the two originals. But I'm guessing the topic was more aimed at "which is your favourite" rather than "which is the best", and considering I haven't played Crystal, I'm gonna go ahead and be biased and say Silver, solely because it was the first Pokemon game I owned myself, and the only one I've really played, unless you count the times where I've played on my brother's to steal his Pokemon. It's not that there's anything specific to the game that makes it better than any of the others (apart from Lugia being available before the E4 :D), it's just that it's the only game of the generation I've really played :p