Emerald isn't an official Pokemon game. It shouldn't be in the poll really.
Anyway, Red/Blue are best for nostalgia purposes. Extremely playable in their day, although they feel a bit dated by today's standards.
Gold/Silver are king. They're the perfect Pokemon titles. Over 200 monsters, great legendaries and brilliant features (radio, phone, day/night etc). It doesn't get much better than this.
Ruby and Sapphire...now here's the tricky one. Brilliant visuals, loads of new monsters, same, addictive gameplay. They had the potential to be the best. But several critical factors ruined it for me.
Firstly, and most importantly: EV capping. Basically, the way the game is built, you're working harder for less when training your team. You max out two stats and the others are left to suck, whereas in the first games you maximized each Pokemon's strengths in each and every area. It's become too over-complicated now. Not to mention the fact that no-one will ever know anything about EVs unless they read up on the games online. :|
Bah, I can't be bothered to list all the other bad points :p. They're good games, but Nintendo have veered away from what Pokemon's all about. And where in God's name is the night/day feature? >_<
Although it'll never happen, the next Pokemon games could be the greatest ever should they decide to go back to the old EV system and add a few new types (instead of natures >_< ). And drop all this E-Reader/mystery ticket rubbish.