I think pretty much every Pokemon game is worth playing, no matter which generation we're in, since we've been following the same formula with pretty much every main game so far, making the experience with the different games similar, but still unique with each. I guess it could kinda be argued that Firered and Leafgreen would be a better choice since they are remakes and have improved on loads of stuff that needed improvement in the originals (ie fixing the Psychic type so it wasn't as OP, adding the new typings, improving the graphics ofc., and additional small stuff), however the older games have a certain charm to them, that still makes them fun to play, even if there are supposed better alternatives :p I often shifted between my Blue and my Leafgreen whenever playing Pokemon (because it was the only games I had really), and despite essentially being the same games, I found both to be a different experience and equally fun. It's not certain that everyone would feel the same way, but I don't think R/B/G/Y are any less worth playing just because they're outdated.