Gen 1: Red or Blue
Gen 2: Crystal
Gen 3: Emerald
Gen 4: Platinum
Gen 5: Black or White
Gen 6: X or Y (Hopefully, Z)
Remakes and major overhauls (B2/W2 and Yellow) do not give a good representation of what that generation had to offer as they change too much or, in the case of remakes, are retreading old ground.
"On my business card I am a corporate president. In my mind I am a game developer. But in my heart I am a gamer." - Satoru Iwata