Only paying attention to mechanics, X/Y is the best gen. Only paying attention to graphics, X/Y is the best gen. As the latest gen, X/Y also has access to the most pokemon. It's finally broken out of the fully 2d (and pseudo 3d 2d that was in B/W) gridlocked nonsense of the past into the current gen, and it's brought with it a bunch of other gameplay changes; level grind has been replaced with minigames to increase your pokemon's happiness (to simulate playing with it as you would a dog or a cat) and training it for battle (increasing its attributes via minigames). You get far more money than you ever need to buy all the potions and items you could possibly want, and you can spend whatever's left over on customizing your clothes or haircut.
The big flaw in X/Y is... the story. It's absolute shit. It contradicts itself several times, even when one character is monologuing his supposed 3000-year history, he contradicts himself on multiple points. The main bad guy is completely illogical (and is obvious as the bad guy the second you meet him), and his entire team of criminal goons are basically the fashion police. It's extra-concentrated stupid. That said, "story" is just kind of what's tacked on to a pokemon game as an afterthought - the training of your team and conquering of gyms/elite 4 is what the game revolves around, and, if you keep playing, you can sink a crazy amount of time into building a competitive PvP team (not joking - pokemon PvP is about as deep as turn based combat goes).
If you really care about story, go with gen 3 - Emerald is the standout from that gen. Amusingly enough, if Nintendo follows the schedule they've been following, gen 3 (ruby/sapphire/emerald) will get remakes some time after Pokemon Z comes out. If you can't wait that long, just play X/Y - it's certainly a very solid product, and all things considered, the "best" gen to date, even despite its awful story.
EDIT: As some other people have already said, there's almost no point at playing multiple pokemon games. It's a nintendo franchise; if you've played one, you've played them all. Sure, a gym leader might have different pokemon. Sure, the legendary available might be different. But the core gameplay is completely the same in every older gen - 8 gyms, 1 elite 4, 1 series of side quests to fight a team of incompetent bad guys doing something stupid and destructive. And grind. A LOT of grind. Typically you'll need to run around hitting every trainer between gyms to make sure you're up to the level you need to be, especially if you haven't memorized the super effective/not very effective pairings. Before the elite 4, you'll spend an extra long time grinding in Victory Road. With this most recent gen, the grind has been scaled back a lot (thanks to the exp share item) so you're free to blaze through the story without really stopping for grindbreaks and then, with access to all the daily quests/pokemon safari/etc, you can spend time crafting your dream team. Or, yknow, playing other games. But really, you will not want to play multiple gens of pokemon in a row - they are not nearly different enough to warrant it, and given how much X/Y has blown them out of the water, if you play that first, you will be miserable trying to slog through the older gens.
