2>6>4>5>1>3
Gen 1 was mostly just a couple of programmers fucking around, and wasn't meant to be that serious, so it just kind of came out feeling more humble and of course buggy. Although some moves were completely broken, the ironic part is that gen 1 had the most number of viable mons in OU. The story was a little short, a little too simple, but it worked for what they were trying to do.
Gen 2 polished up Gen 1 nicely, and it is admittedly the gen with the most fan service, even HG/SS has the GBA sound mode, and follower pokemon. Gen 2 also is when Gamefreak became more self-aware about serious battling and added in steel and dark to hamper psychic type's reign, and added some much needed work to the bug type.
Gen 3 felt the most rushed and it was the point where the series for many began to feel like a cash cow, and by gen 3 much of the original audience had gone. The designs of the pokemon got much more polygonal, with the formula seeming to be take a geometric shape, add a weird patch of color, and add angry eyes. They also dropped the ball with the music. Notable improvements though included secret bases, and abilities. That was about it.
Gen 4, like 2, was about polish, and made the series feel cohesive again. It explored older mons, added a little more mythology, and brought some better tracks to the music. The map was a little forgetful, but overall it was a needed step down from Gen 3's overambition to reinvent the series. The physical/special split was also much needed.
Gen 5 is very similar to 3 and 1, heavy experimentation, which a lot of failed. The greater points were the introduction of Global Link and the best story the series has seen to date. It was a little bit of a graphical mess, trying to find legs in 3D, but still stuck in sprites. They absolutely should have waited to try going that 3D heavy until gen 6.
Gen 6 still needs time to settle, but seems to be another well polished even gen.
