Actually, I wouldn't go any further back than the 3rd generation games if I were you. The mechanics from the first two generations were dramatically revamped in the third generation, and I don't think you'd get a proper feel for how the series plays now by playing Red/Blue or Gold/Silver.
If you have access to a GBA or can play GBA games on your DS then I'd recommend Emerald Version first. It was my first Pokemon game and it showed me the ropes of the series. The difficulty is just about right as well (ok the Hoenn Elite 4 kinda blow but whatever). Plus I love the presentation in the game, the soundtrack and battle animations are pretty awesome, and it has a wacky plot about doofuses trying to flood the world vs trying to cover the world in lava.
If you don't have access to that, though, then I'd recommend Black/White 1 or 2. Depends on how much emphasis you place on story in games. If you do, then Black/White 1 is perfect. It has a plot unlike any before or since, one that really goes into why people battle with Pokemon. It's sorta a deconstruction of the series as a whole, now that I look at it. But if you don't really care about that and just want to fight, then Black 2/ White 2 all the way. It's not quite as deep of a plot as the first ones, but it compensates for it by being the best damn iteration of a Pokemon game I've ever seen. Everything is superb; battle animations are fluid and impactful, it has Pokemon from all the generations so you can create the team you really want instead of being restricted to the Unovan Pokemon like the first ones do, it has a ton of stuff to do on the sides (my favorite's PokeStudios), it's the best game on the DS in terms of visuals and audio, and the most important part: it's actually pretty challenging at times. Several parts I was put into a corner by either the gym leaders or Team Plasma, and when I was fighting Colress I almost died (You don't understand, that NEVER happens to me in a Pokemon game!) Plus, after the main quest there's a crapton of really strong fighters to battle, and the Unovan Pokemon League on Challenge Mode is the hardest thing I've done in a Pokemon game (particularly if you're trying to do the Type Medal challenges like I am). Also there is the Pokemon World Tournament, where you can enter competitions with all the gym leaders from all the regions. Freaking amazing!
Actually, screw all that. Just get Black/White 2 now.