Gen 1. The only others i played at all were 3 and 6 and I'll be honest here, the Pokemon designs went way down hill after gen 1. Those designs are brilliant but most I've seen from later generations tend to range from mediocre to terrible.
Gen 1 ( red blue yellow)
Gen 2 ( Gold silver crystal)
Gen 3 (Ruby Sapphire Emerald, Fire red Leaf green)
Gen 4 (Diamond Pearl Platinum, HeartGold SoulSilver)
Gen 5 (Black White, Black 2 White 2)
Gen 6 (X Y, Omega Ruby Alpha Sapphire)
Gen 1. The only others i played at all were 3 and 6 and I'll be honest here, the Pokemon designs went way down hill after gen 1. Those designs are brilliant but most I've seen from later generations tend to range from mediocre to terrible.