I went with gen 3 because of the great pokemon designs and great mapping.
The first two games were great, yet limited in the design and mapping on the gb/gbc.
The 4th generation had great pokemon designs in the pokemon lines it introduced; however the enormous amount of new evolutions from past generations was unneeded and have a lasting negative effect on the series onward. Lickilicky, Tangrowth, Togekiss, Magnezone, Yanmega, Mamoswine, Porygon-Z, and Probonose were not great additions. Froslass, Dusnoir, Weavile, Roserade and a couple eeveelutions was more than sufficient. Though, I am on the fence amount electrivire, and magmortar, which should have included a Jynx evo. Overall, there were too many of them, and most of them were these bulky, poorly designed, and unneeded additions. Now, for the mapping, it was a mess; a game doesn't have to be linear, but this game was very diffuse and overly-complex in comparison to the other games. This is by far my least favorite generation.
B/W was a great addition to the series with original designs and GREAT mapping. There were an abundance of new features that made the game feel rejuvenated and fresh. The big criticisms I had was the Pokemon Musical, I prefer the third generation contests, and the story-line of the evil team; I mean, yes, forcing a Pokemon into captivity of a Pokeball doesn't seem so nice when you really think about, that is why GF ignored this type of plot because it does call into question some viable ethical issues. Overall, a good game though!