Well, let me elaborate a bit:
1st gen - The original 150. It's basically the building blocks of what we see today. You get a wide variety of Pokemon from tanks to walls, sweepers, to useless pieces of junk. Perhaps it's nostalgia that makes the original gen my fave.
2nd gen - We get introduced to a lot of OU who still own Pokemon battles even today, like Skarmory. :) There's not much to say about the 2nd gen, but they were super nice additions to the original 150. Lots of babies, though D:
3rd gen - And I DO agree with Burgers Rule, how we get a lot of the powerhouses. But, a lot of those are ubers, which (I think?) are banned from Netbattle? I don't know, it's been a while. Nice pokes, but some are getting repetitive in terms of looks... and names. (Spheal and Seel? What the heck?)
4th gen - Okay, I know it's almost been a year since D/P came out, but they still feel too new. Some of the typings are interesting, but some are awful (Rock/Steel? Wtf? I liked Frosslass, though, Ice/Ghost is freaking cool, imo). Besides, there are too many Legendaries and evos to previous gen. pokemon. I don't feel like we're adding too many ORIGINAL pokemon to the already massive pool. Besides, it's too freaking stressful to evolve them (evolve by holding an item... and you can find this item...?).