My votes will go to
#1. Supreme Commander - This game rapes hardware but it has such an amazing mod system, a superb queuing system, coupled with strategic zoom (its better than sliced bread), and large scaled battles that can last for hours on end, this game is always sweet action. Sometimes finding the right mods can really help people enjoy a game and this game is extremely mod friendly.
#2. Total Annihilation - Shares much of Supreme Commander's excellent features and for about an entire decade was considered to be the #1 RTS and was most certainly the most modded game ever created.
#3. Sacrifice - Very unique First Person RTS game, mixing spell casting, summoning, soul stealing, and some freaking crazy cool creatures to command. Down side was slow movement for map size and ridiculous difficulty of the latter levels of single player. Also the game came with the map editor used for the actual game campaign (you could literally edit the official game missions with it if you found it to be too difficult, which i did...)
#4. Starcraft - 3 unique races, unmatched balance between the factions, intense LAN matches, the works. Certainly could have used some easement on the micro intensiveness but it was certainly fun for a good while.
#5. Command & Conquer: Generals - Another good RTS, 3 races, a fair bit of diversity, a ton of fun on LAN, fairly simple game plan, pretty much all units are useful throughout the whole battle.
