MegaCharizardX

I have Pokémon X and Pokémon US, but I like X far more than US.

It´s because X has a much better difficulty curve. Also, it has a kinda easy storyline, and it´s easy to play. The menus are easy to use and it has many more funtions than Black 2, White 2 or previous. Not that I played B2/W2, but I´ve seen Lets Plays of it and it did seem a lot more complicated. X has a much easier gameplay, with enough info in the texts of NPC´s to know what to do, and leveling up is easy, due to the Exp Share, but also because each time you unlock a new route, the wild Pokémon found there will be (expected to be at least) around the same Level as your Pokémon, so if you have taken the time to train your Pokémon up to around the highest level that the mon will obey you (which is around 35 after you beat Viola, and around 80 when you beat Valerie) it´s a piece of cake. But of course having overleveled Pokémon is not much fun as the battles in the storyline won´t be a challenge, and challenge is what we want. US is for me not much fun, because the difficulty raises too quick during the storyline because the NPC´s mons´ level raises quicker. And because of the fact that your Pokémon gain much less Exp for battles with wild mons than in X, the difficulty curve doesn´t quite fit. Also, I personally find the lay-out of the menus, the speech bubbles and all not suiting. It´s too colorful for me. Also, the storyline in X is a lot more interesting if you ask me.

I just bought Pokémon Y, yesterday actually, and I can´t wait to play it!

In my opinion X or Y are the best choice. The next one on my list is OR/AS just because of the Mega Evolution.