Even if we got to gen 15 and I actually started to think Nintendo was getting unoriginal, I'd still be excited. I'm always happy to see what new stories they come out with, what new Pokemon they have, and what the newest technology can now do with a game. I'm astounded by the posts on tumblr showing gameplay from Red, Yellow and Blue side by side with gameplay from X and Y.
And I said before I have yet to find one that isn't high on my favourite list of games.
For me, personally:
1st gen - Just a big nostalgia factor, really
2nd gen - ...I'll admit I'm bigger on the anime than the games so I'll set this one aside
3rd gen - Had my first favourite team, also piles on nostalgia as it's the first Pokemon game I had where I beat the whole game by myself, didn't have my brother do anything
4th gen - Had the best personally for gameplay
5th gen -Black 2/White 2 had the best in continuity, story plots, etc for me
I'm all excited for gen 6 now.