I'll be honest: even though I've played through most of the Pokémon games, I don't really remember most of the villains I've faced over the years... with one exception, Giovanni.
TheJWittz here dives into the Team Rocket boss and shows us exactly why he's so memorable -- aside from giving us one of the best twists in all the games (who could have guessed he'd be a gym leader back in Red and Blue?), Giovanni is also the source of a lot of theories and speculation. Is he, for example, Ash Ketchum's dad? Did he kill himself? Is it possible he's actually the good guy?
We might never know the answer to this stuff, but that's OK. Half the fun is the speculation.