My no. 1 pic is Netstorm, which I still play www.netstormhq.com
Reason being it is still totally different from any real time strategy games in that offensive units are static and are built of a complex series of bridges. Sure it has a difficult learning curve, noobs get owned hard and the graphics are **** but after a few games it's actually quite fun. Not to mention the games are quite fast and you don't have to spend hour upon hour playing the game.