In my humble opinion, the five generations seem to follow a pyramid similar to that of the Dramatic Structure:
The intro/exposition is the original games. They were the beginning of one of the greatest series to hit the gaming industry. They introduced the first 151 Pokémon, they introduced the battling system, and everything else that Pokémon is known for at this day and age. They were definitely a group of solid Video Games. Although, they are definitely not the best. They definitely set up the rest of the "plot" up to Generation 3, I just find that they're too basic. Not easy per se, but dull. Uninteresting. There really wasn't a lot going on in those games compared to later ones. Also, most of the Pokémon, though neat, are relatively basic.
The rising action is the second Gen. These games introduced many new Pokémon to the series, most of which I prefer over the first gen's Pokémon. They were more interesting and creative, and their designs were a lot cooler for the most part. There was also a new region introduced, which I personally like a lot better than Kanto. And on top of all that, this generation introduced a day/night system, which I thought was really cool, if a little annoying at times. These games are, overall, much better than the originals in almost every way.
Then we reach the Third Gen, AKA the Climax. This is, and always will be, my favourite generation. Although they ditched the day/night system, there were tons of new aspects introduced in this generation, notable the Battle Frontier, and the addition of remakes of classic games. Once again, there were tons of new Pokémon that were introduced, as well as a new tropical paradise type region, which is actually my favourite in the series. The Pokémon that were added are some of my favourites, as both my favourite legendary and favourite Pokémon altogether are from these games. The games were a huge graphical improvement over the games from the previous generations, which made playing the game a lot more fun because they looked nicer. The addition of two villainous teams, instead of just one, makes these games unique, even if the two teams do have rather outlandish goals. And then there's the aftergame. Ruby and Sapphire lacked a really big aftergame, but Leaf Green, Fire Red, and Emerald all greatly expanded their respective aftergames. FR and LG added a completely new region after defeating the Elite Four, which no other game in the series has done (unless you count the Generation 2 games, but that was simply a return to a previous region.) So, all in all, these games are great! Definitely the peak of the franchise, in my opinion at least.
The Fourth Gen, while still a group of solid games, is where the franchise started falling imo. Don't get me wrong, as I absolutely adore the fourth gen games, I just didn't think they lived up to the glory of the third gen. I like Sinnoh, and I like most of the Pokémon that the fourth gen brought to the table, but I like the third gen better overall. However, I was ecstatic when I found out that they were bringing back the day/night system, and even more overjoyed when I found out that Gold and Silver were being remade. Just like FR and LG before it, they were 2 great remakes, and definitely improved upon what G&S and attempted to do. The Fourth Gen followed the example of Generation three with an extended aftergame, but I didn't really feel like the games brought too many new aspects to the franchise. Overall though, these were a big group of epic games; they just weren't as good as the generation before it.
And then we get into Generation 5, AKA the Conclusion to what we know as Pokémon. As much as I like the Generation 5 games, I really think they are the worst in the series. Though there were a total 156 Pokémon added this gen, most of them were very unoriginal and, quite frankly, boring. Unova is also my least favourite region, and I am really not that much of a fan of many of this generation's legendary Pokémon, bar Genesect. Although, I must say that it had one of the best stories in the franchise. Team Plasma has the most realistic goal, and they were actually relatively smart. Unfortunately, I didn't like their attire... Like, at all. But that's a minor complaint, and doesn't effect the game as a whole. But unfortunately, the games in this generation did fail to live up to all the previous games, which is a shame because there was a lot of potential. I think Nintendo was just running out of ideas and needed a jumpstart.
So yes, I think generation 5 is the bottom of the pyramid, which means the games have reached their decline. Luckily though, Pokémon X&Y certainly looks like a step in the right direction, and may be what the series needs to regain the awesomeness that was seen in earlier games. I am definitely excited for October.