I would wait for the new games coming out soon, Omega Ruby & Alpha Saphire. Their remakes of Gen 3, my first Pokèmon game. Their is going to be a lot of new mega evolutions and it should be really good.
I got X & Y just a few months ago and it's really not worth it. The story mode was boring and I find the online on it really annoying. (The Battle Spot only let's you use 3 Pokes at a time and everyone on there just uses a full team of legendaries against you.)
I recently got into pokemon showdown, if you don't know it's an online battle simulator. You can pick out any pokemon you want so basically you have the ability to make whatever teams you want and battle people, it's pretty fun. Just google it if you haven't heard of it
XY are accessible and available...but pretty subpar in the grand scheme of things.
I'd heavily suggest Heart Gold, Soul Silver, or Platinum as your first game, but keep in mind that there's a bit of hassle in transferring from gen 4 to 5.
You can do Pokemon Black/White 2, they're pretty good. Make sure it's the sequel though, the original Black/White are just awful. Unova is just awful... but they didn't do any remakes last gen, so it's all you've got for gen 5.
Alternatively, you can just wait for ORAS, which are remakes of the Gen 3 games, two of the best in the series by a large margin. They'll be fantastic.
Currently playing: Pokemon Y (Name: Ensei), Xenoblade Chronicles, 3DS FC: 2277-7562-5299 FS: Ghost @ Drfiblim, Lampent, Pumpkaboo Steel types! MegaMeta!