Fynn pretty much covered it. I love Red and Blue, but gameplay wise they're pretty outdated and are prone to annoying glitches. Fire Red and Leaf Green are as close to the originals as you can get without it scaring a newcomer off. I'd still recommend Yellow though if you like the anime at all since it follows it pretty closely with Pikachu and stuff.

Gold/Silver set the tone for the rest of the series and Crystal was the first game to introduce playing as a woman, so they're pretty neat.

We all know the third game is going to be called Pokemon Z, Fynn.