All the games are pretty self-contained except for BW2, so... If you're interested in the story and recognizing all returning characters, I'd suggest Fire Red/Leaf Green even though I haven't played them myself. The original Red and Blue are too broken by today's standards, but they were fun to play in their time. Although ultimately it depends on what console you own. I recommend starting with a game you can run rather than buy an old console just to play old games. Just don't start with BW2. Any other game is fine.