To be honest, I like all the generations.
Generation I was where it all started. It was what made Pokémon what it is today. Plus, it's super nostalgic.
Generation II, I sadly haven't played much of, it it added some cool new features and upgraded Pokémon a little.
Generation III was another upgrade in which they also added some cool new features. Plus, Hoenn is awesome, and the soundtrack is amazing. It also set the stage for an amazing remake. Plus, contests!
Generation IV, personally, was my first Pokémon game, and what got me into the games. It's super nostalgic for me.
Generation V...well, it had some cool and hilarious Pokémon here and there. XD
Generation VI is where you could do so much more with Pokémon. At least a step up from Generation V, definitely.
Generation VII, the current one, I haven't played much of, but it's a Pokémon game like no other. The core gameplay, of course, is the same, but everything else is very unique and gives the game its own flavor.