I'd suggest either Black, White, B2, or W2. Black and White are basically series reboots, and B2 and W2 are in my opinion the closest to perfect Pokemon games that are out there. All, however, are fantastic games that are fun, feature-rich, and feature a good amount of difficulty without being too grindy. 6th gen is ridiculously easy and kind of shallow, so I'd recommend you play those later.