gold/silver/crystal, after a couple of false starts (hated two of the three starters, only the first generation avoided this), my crystal file clocked in at about 350 hours and it i genuinely the one i had the most fun in, it fixed all the problems and needless busy-work that the first one contained and built on what happenned and where we went in the first one. since then, almost every game has been completely standalone. also, this was, i think, the last pokemon game before it became COMPLETELY ridiculous (have you seen the new pokemon? there's one pokemon that is basically a rubbish bag for god's sake). not only that, but it was the last one to feature team rocket as the main villains (i don't count remakes, never liked them, never will, unless the original was awful), and i loved team rocket, they were completely ineffetual, but i don't care, the plots since then have either been kinda shoehorned in (although i've heard the plot for black/white is pretty good, but i completely stoppe caring during diamond/pearl) or even dumber than team rocket (flood the world? evaporate the oceans? THESE ARE NOT GOOD IDEAS!!!)
anyway, my favorite is gold/silver/crystal and i hope i've adequately explained why