I'm nearing the end of my second EW playthrough and first-ever classic run. Other than a couple freezes I've had a great experience. This is an all-time favourite for me. It's thrilling and challenging and fair. It can frustrate me or make me feel like a badass. It's a strategy game that plays great with a controller.
Enemy Unknown (PS3) was my first XCOM experience, and as good as it is, they added so much to Enemy Within! The MECs, Gene Mods, Medals, and enemies give you more to play with. The missions associated with Covert Operations, Site Recon, Operation Progeny, Operation Ashes and Temples, and the Slingshot DLC beef up the mid-game. The many new maps add much-needed variety.