Warzone 2100, by Eidos Interactive, is by far, the best real-time strategy game I have ever played.  I believe what makes it so good is that it is so completely different from any other game out on the market.  For starters, there are no pre-made units in Warzone apart from the construction vehicle.  Every unit you build in the game, you custom-make yourself.  There are over 2,000 possibilities of different units that can be made.  The way you accomplish this is by researching different aspects of vehicles or buildings.  After you research something, you can use it in the design of any vehicle or aircraft as many times as you want.  The reason why I like this so much is that instead of trying to settle for something because the exact unit you want just isn't available, you just make it and build it, it's as simple as that.  Another thing about Warzone that is different from any other strategy game, is that you can play the game from any view you want.  All you do is right-click anywhere on the screen and drag you mouse to move the camera around.  You can also zoom in and out as you please.  This is very advantageous in a hectic battle because you can look at your base from an enemy's perspective and see any weak points there might be in your defensive line.  You can adjust the camera anywhere from 90 degrees to about 2 degrees 360 degrees around giving you an almost unlimited number of vantage points from which to fight your opponent and if the view you have doesn't let you see something (eg: behind a tree or hill), all you do is rotate the angle around you can see.  Also, as you scroll, if the camera is at a low degree, it will automatically adjust to the terrain so you wont end up going through the middle of a mountain just trying to scroll.  Another very impressive feature about Warzone is that it is amazingly realistic.  Units and buildings have a very high chance of hitting an enemy if they're on a hill and units firing up trying to hit something that is elevated have a very hard time at doing it and even if they do manage to hit it, they do significantly less damage then if they were on even terrain.  This can be used to your advantage since most bases are already on high ground but it also makes it very hard to hurt an enemies defense at their base.  One disadvantage though about Warzone is that Unlike most other strategy games with two or three resources required to collect, there is only one, energy.  This could prove to be good because you wouldn't have to defend multiple mining locations but I still don't exactly care for it that much.  Overall, I think this is the best real-time strategy game out on the market and a must-buy for anyone.