1 - Gen 2
A lot of content, seeing how Kanto changed over the last three years (best post-game ever) , a Pokédex that wasn't imbalanced, and wasn't yet too big to complete. Putting in completely different sprites for most Pokémon in all three versions was a nice touch. Not to mention, my favorite soundtrack in the series (unfortunately, the remakes butchered some of the songs) . It is also one of the three adventures with a Victory Road that I don't hate.
On a side note, this generation put in place the template for the future stories that became quite old as time has passed: you see the evil team put stuff in place at different points in your adventure, then, right after you get your seventh badge, the evil team makes its move, most often involving the legendary version mascot (not the case here) you battle them, the team is disbanded, and then the final badge and the Pokémon league are more of an afterthought. We'll see this template over and over again in the future.
2 - Gen 5
Two different and new adventures in one generation, who would have thought? :) While more story-driven than the rest (with the unfortunate side-effect of constant handholding) , it gave us what I consider to be some of the most interesting characters in the series, be it rivals, gym leaders or the evil team. The template that I discussed above, while visibly attempted both times, is not actually used. Also, Black and White versions have the most differences between versions I've seen in Pokémon games (Opelucid City, Black City/White Forest...) .
I especially like Black 1 & White 1 for two reasons: completely new Pokémon that, on first playthrough, made me feel like I was discovering the series all over again, which felt really cool, and of course the story, including what I consider to be the best ending in the series. I also liked how the evil team's motivations and goals were a bit more, can I say, realistic in these games? Instead of looking for some Pokémon and use its power to change the world, they manipulate people.
Black 2 & White 2 have lots of completely optional areas, which is cool. Also, I like the story that is more classic than in Black & White, but putting it in the context of what happened in the first games IMO really works. They also contain the second Victory Road out of three that I don't hate.
3 - Gen 3
The first generation after the reboot. The games turned me off at first, because not knowing what a regional Pokédex was, I thought they completely threw most of old Pokémon out, but after a friend showed me a complete National Pokédex, I was relieved.
While at first I was disappointed by not being able to go to the other regions, upon replay I realized how big Hoenn actually was, so now I don't mind. I also really liked how roles of both evil teams were reversed depending on the version you're playing, and how Emerald managed to combine both versions of the story.
4 - Gen 6
Why so low, you ask? Well, I barely care about the leap to 3D, and while I do like nostalgia, these games really overdid it (reusing the map of Viridian Forest, really?). In terms of story, I recognize that Gen 5 was a tough act to follow, but what we have here is a story that combines the worst of the other stories: constant handholding as seen in Gen 5, characters that give us no reason to care about them (the only ones I did care about that I can think of are Emma and, strangely, Lysandre), the evil team is back to having ridiculous goals, and the template I described above is back in full effect. There's not much exploration to be done here, and the post-game is almost nonexistent.
Oh, and the Pokémon Transfer IMO should have been an offline feature. I described in another thread how I think it could have worked while having the hack checker up to date etc, I'm not going to repeat myself here.
On a positive note, the graphics are nice to look at, the soundtrack has a few memorable tunes. The Exp Share is a godsend to people like me who hate grinding, and this is also the third adventure where I don't hate the Victory Road. But other than that, Gen 6 is... just there.
5 - Gen 4
While I do recognize that these versions did bring a lot changes and adjustments to the gameplay that I enjoyed, the overall feeling I got from Diamond and Pearl was "Gen 3, part 2", minus the interesting alterations in the story between versions. I also thought the stretch between the second and third badges was painfully long, the very long save times and the painfully slow battle animations were inexcusable, the difficulty wasn't all that well scaled, the requirement to see all the Pokémon in the Regional Dex to unlock the postgame was an incredibly pathetic attempt at making the game longer, and, let's be real here, the legendary Pokémon were ridiculous (creators of time, space and antimatter, and GOD, really?) .
Platinum tried to redeem this gen by fixing a lot of these problems (and I really liked what they did with the lighting in some areas, like the caves) , but IMO it was to little too late. Also, a moment I found aggravating in Platinum: you know how I feel about event exclusives, I've been rambling for a while now on how they shouldn't have been included in the National Pokédex at all... well, having to see Manaphy, a Pokémon that can't be caught, in order to proceed was just plain trolling on GF's part.
6 - Gen 1
I understand, this game was the one that started it all, but it aged horribly. A lot of things were wrong with those games: horrible graphics, a completely unbalanced ensemble of Pokémon (Psychic owning everything else, single Special stat), no breeding (a big deal if you're trying to complete the Pokédex) , the bag that holds everything in one pocket and finds itself constantly being filled up... just play the remakes, that's all I'll say.