I'm an old school player, played and completed them all however my first save on Pokémon red is still there, caught 149/150 pokémon, never could catch kangeskan in the park.
In my opinion, I can't vote on this poll, I can only say that
First gen is cool, because it was the first games, it was a little effed up.
Second gen was lack luster, despite being precious metals. 4th gen remakes fixed some stuff but not enough.
Third Gen was just a waste of time, I hate Ruby and Sapphire, Emerald was a slight improvement, the remakes of Gen 1 kept me in Pokemon
Fourth gen was nothing short of amazing
Fifth gen took a lot of awesome stuff from fourth out, but it's still really solid and fun, which I didn't get from third gen as much and second gen is fun to an extent but the wild levels are just all way too low... the best training spot is level 27s in the last town, that's just sadly pathetic...
Gotta be Red/Blue its the original and what i played as a kid plus they haven't changed the other games that much from the way it was back then, so you have to pick what started it all.
Gold Silver and Crystal was my favorite, up to Ruby and Saphire it was good, Diamond and Pearl was playable, but Black and White was just terrible. Plus one of a kind pokemon add up to 30 now. After gen 2 there were just too many pokemans to capture.
i loved read/blue and gold/silver so i really can't choose between the those 2 generations in fact i still play those when i get bored of other things, and they're still fun :D
First and second gen. Ive avoided the rest until I grabbed White the other day which I havent played yet as I had a nostalgic moment and heard these are the best since the originals over a decade ago.
I'm going to say Gold/Silver. If I add up all my hours spent on Pokemon I'd estimate 1500... Mostly on Sapphire and Ruby, me and a friend got addicted and were driven to complete our Pokedex. We succeeded at completing the Hoenn 'dex.
Of course Red/Blue is classic, but there are so many little things I love about Gold/Silver, the battle with Red, Lance's theme, how Koga goes from Gym leader to Elite four, etc. :)