I feel like Gen I was a great game and has those nostalgic feels for sure - but there are so many features which were introduced in later generations which just blow it out of the water.
Gen II & III are probably my favourites in terms of pokemon design, graphics, and story-line.
Gen IV was good, but I wasn't in love with it like the previous games. Gen V was a disaster, I didn't finish my version of White and I never bought W2. The graphics were appalling and I simply couldn't look at it. (HAHAHH, I know, such a trivial reason.. But I felt like the graphics were a step backwards).
Gen VI and VII have been amazing. I love the new design and movement style - my only complaint was the excessive cut scenes in VII.