I have to go with Black 2 and White 2. As if I haven't explained enough reason why I love this game, here are a few:
-Lots of Pokémon to choose from and play through the main game with. The possibilities for teams are nearly endless.
-It has so much to do. The Black Tower/White Treehollow, getting to battle all the Gym Leaders in the PWT, and lots of powerful trainers to take on and cool places to explore make for a great post-game. And they actually try to motivate you to complete the Pokédex for the first time since Emerald with some nice rewards.
-Great conveniences such as the free space, the fact that you can put most menus on Select, moving items from one Pokémon to another, and being able to spray Repels again right away. Those are all great.
-VERY good visuals. The battle scenes, for example - animated trainer sprites, the grass moves in the background, caves and forests look much more lively, and the Elite Four got some epic presentations. Oh, and that brings me to...
-The battle with Iris. While she's not the hardest Champion ever, she still makes for an awesome, AWESOME fight, especially on Challenge Mode. I've always loved battling that Haxorus. And the credits afterward are just so moving, because it shows you partying it up with the Gym Leaders as you head home.
I could go on, but these are just a few reasons as to why I love Black 2 and White 2 and that they will NEVER be forgotten by me. Ever.