Age of Empires II HD is a classic RTS game brought to modern times. The HD edition has upgraded graphics and comes with a multiplayer mode attached to a really good, loyal community.
If you have never played AoE, you truly are missing out and should pick it up. If you have, it's time you get back into it. The DLCs add more civilizations, new maps, and new campaigns. The HD version also features a better AI and mod support. At 75% off during the sale, it's a necessary buy.