It's my honest and sincere belief that Civ4, when played with both its official expansions, is the perfect strategy game. I have a lot of trouble getting into most 4X games, even turn-based ones, but what I think makes this game far better, than, say, Galactic Civilizations 2, is that it's complex enough to give rise to various strategies, game styles and outcomes, but you're still presented with enough detail that you can look at your civilization, its cities and its individual units and find out exactly why they're unhappy, or unsuccessful, or unprofitable.
Games like GalCiv2 hide the formulas that govern your civilization under the hood, and playing them ends up feeling more like SimCity than Risk or chess: you simply find yourself responding to events as they happen rather than formulating long-term plans and goals. Civ4 throws that sort of design out completely--it's detailed, but not ridiculously so (like, say Master of Orion 3, where you feel like you're playing a spreadsheet), and knowing that its design is good, it doesn't feel obligated to hide the details.
This is a great game.