Pokemon games are ones of the most played and sold games on consoles history. In 1996 first Pokemon game is published by Nintendo and developed by Game Freak for Game Boy and continued  on each console generation of Nintendo. loveroms

We can count the playable  Pokemon Games generations free and unlocked  with online emulator so:

Generation I (Game Boy)

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Pocket Monsters: Red and Green

Pocket Monsters: Blue

Pokémon Red and Blue

Pokémon Yellow

Generation II (Game Boy Color)

Pokémon Gold and Silver

Pokémon Crystal

Generation III (Game Boy Advance)

Pokémon Ruby and Sapphire

Pokémon FireRed and LeafGreen

Pokémon Emerald

Generation IV (Nintendo DS)

Pokémon Diamond and Pearl

Pokémon Platinum

Pokémon HeartGold and SoulSilver

Generation V (Nintendo DS)

Pokémon Black and White

Pokémon Black 2 and White 2

Source: Wikipedia