Operation Flashpoint is the best game that has yet been made. For any one who doesn't know, it's a modern war game that's a cross between a FPS & a squad based strategy. The maps are not mazes, like the ones in such gams as MoH, Halo & Call of Duty, they're small island of around a hundred square kilometers of land, scattered with towns. There's Three official campaign, a couple of dozen official single missions, a hundred or so official units and an unlimited amount of free third party units, missions, campaigns & maps. And Just because the third party stuff is free and, well, third party, doesn't mean it's lesser quality than the official stuff. There are alot of very talented addon makers for this game
Fallout 1,2 & tactics are the best RPGs.
The best RTS would have had to have been Total Annihilation, but I haven't played it for ages.