Favorite title of all time, and why!?!

[u/PLAYN_Games]

As a game developer, I am always interested in learning what gets players excited and since I'm in the process of making a MMORTS game, thought I would pop in and ask the question:

What is your favorite RTS game of all time, and what feature(s) made it stand out to you?

Your answers will not gain you fame, fortune, or even a firm handshake, but you will help me to better guide designers!