Its not even a contest. The first generation, by far. The others can't even hold a candle to the original 151. The others aren't bad, but I feel all the best ideas were used in Red/Blue and everything else was just an afterthought.
Wait, do you mean Pokemon games or actual generation of Pokemon species? Either way, its Red/Blue.
I'm 18 so at the time the first generation of pokemon was right in my wheelhouse when it started (along with my whole school), as was the gold/silver era but I prefer the original run. I've seen my younger brothers playing the newer pokemon games and the new pokemon are complete nonsense. On a side note, pokemon red through gold were and still are great rpg's in their own right.
Third gen, ruby and sapphire were amazing games. Sure I spent a lot of time as a kid playing the first 2 generations but they just haven't held up over time. Terrible artwork, broken leveling systems, very basic battle mechanics. Ruby and Sapphire made the series more complex and customizable and looked great visually. I've spent more time on Ruby alone than I have on r/b/y/gold/silver/crystal combined.
4th gen morphed me into a competitive player but I did not like Sinnoh as much as Hoenn, just not as memorable. Also there were only TWO FIRE POKEMON species in the entire zone which was a bitch.
The original generation might hold the most nostalgia value, but Gold/Silver are good too. The pokemon that were introduced in G/S may not have been as good as the original, but it felt like a solid expansion upon them. G/S also added a bunch of new features, and there were 16 badges. Every generation since Gold/Silver/Crystal has felt like a rehash though, and I really haven't cared about the new pokemon.
" Red/Blue/Yellow. Obvious for anyone who really likes Pokemon. "False. It's obvious for anyone who's really nostalgic about Pokemon.
I don't have a favorite. I like them all equally. Perhaps the 4th one a little less than the others, but that was still good.
" Gold/Silver/Crystal. Duh. Probably the only generation to add wide-sweeping changes to the franchise for the better (i.e. not contests), and the creature designs still didn't suck. Those who say otherwise probably never got past Red/Blue/Yellow anyways. "I said Red/Blue because I thought the OP meant Pokemon creatures, not the games. So have a lot of other people in this thread. I would definitely say Pokemon Gold/Silver were the best in the series.
And the Gold/Silver designs did suck compared to the first generation. Not as much as 3 onwards, but they still marked a the start of their downfall.
After playing Soul Silver I'm surprised how much the series 'evolved'. I mean sure fundamentally it's very similar but I must say I like the pacing of Ruby and Diamond much better.
I'm picking Ruby over Diamond though, the weather changes due to the legendaries made them feel more 'legendary'.
There have been some interesting Pokemon that have come out in the later generations, but they are diamonds in the rough. Originals are still the best, but I can appreciate the desire to expand the number as people get used to the current ones. Its just a shame that for the most part new ones are just re-hashes of old types with new faces. Granted its not like they can introduce too many new types, but different faces on the same team of 'types' doesn't really make the new generations any more fun.