I think in order of my liking of each generation:

No. II - Large world, improved many of I's faults and oversights. Interesting new type and good designs.
No. IV - Should have been II's spiritual successor. Again, good story, nice pokemon, good features etc.
No. I - Is the original, and is the only game where you "might" just catch em' all.
No. V - Haven't played it, but the pokemon designs seem farfetched and I'm not compelled to buy it.
No. III - Didn't float my boat. The water to land ratio was just huge! Designs seem blocky.

What I didn't like was No.5 bucking the trend of the "third game" of the generation as well. Red and Blue were joined with Yellow, Silver and Gold were joined with Crystal and it goes on. Why did Black and White decide rather than just one sequel (grey?) instead they go with Black Version 2 and White Version 2. @#!?'#!!!!!!