Silver/Crystal – Good mix of numbers, places, size and new Pokemon. It felt like what a sequel should be: a continuation of what came before. In this case, part of that was allowing you to go back to the place you were; only a few years later, and a very disappointed Blue later.
Blue – The game that got me to fall for the genre in the first place. I borrowed my cousins version to help him through a certain ghost and then didn’t want to stop. Eventually got my own GameBoy Pocket Color and caught 148+ Pokemon (I still remember to this day), had to make a few trades to fill out the Pokédex but man was that fun at the time. That may have been the first “social” I ever played now that I think about; couldn’t beat the game without help from other people.
X – This may be the best of the series from an overall perspective: story, art, new Pokemon, game-play. But it doesn’t shine in any situation (except one), it’s just “good” but it’s at least “good” in all the categories. There’s a good chance that in my brain this is really tied with Black, but something about X/Y got me to play over and over again- Wonder Trade runs, small batch runs, single Pokemon runs. On the replay scale this is far and away the highest for me; but I think only because I enjoyed the farming for Crystal so much.
Black – Since Blue all the stories had been the same; until Black! I wanted to get through the towns and the dojos just to see what was coming next in the story. The Pokemon also felt more like throwbacks to the original than the two before it, which helped a little bit. Some quirkiness to the new camera aside the visuals were my favorite since Silver.
Pearl – The way the story and gyms progressed I got really bored for some reason. Got to the point where I could start breeding and got a little overwhelmed with how many Pokemon I had to breed to get. Ended up playing this more after Black came out to work on migrating Pokémon over to it.
Sapphire – The graphics seemed the worse in a weird way; too close to GameBoy without enough jump to GBA. For as much as I liked my GBA I could never quite get in to this version of Pokemon. In thinking about it I’m pretty sure Silver just raised my demands so high that there’s nothing Sapphire could have done.
Without talking about re-releases and remixes, etc that’s where I fall on the Pokéman liking curve.
Updated 2016-04-09: Added in X/Y