C&C3 would be my recommendation if you're buying from Best Buy. I also think C&C Generals and the Zero Hour expansions are amazingly fun, but those are more of bargain bin games by now. If you decide to try Generals, search for mods for it, there are a lot of fantastic ones, including some good realism mods. The Pro:Gen mod has been out of active development for some time, but is still nice. Its modifications are in the same vein and style as the original game, nothing out of the ordinary. The author has made several mods for many C&C games, usually they do things like bring back units not in the final release (but still exist in the asset files), that sort of thing.
Also for bargain bin games, try out Red Alert 2, and its expansion, Yuri's Revenge. On their own they are great games, but with the DeeZire mod, they're
amazing. You need Yuri's Revenge to run it on XP or newer, however. It's by the same guy that makes Pro:Gen, and adds a
lot of content. Originally it started out just adding unfinished units from the game's assets, but by the final release it has ended up more like an unofficial expansion. It's incredibly fun and dynamic.
You can find both mods
here.
Bassoon, n. A brazen instrument into which a fool blows out his brains.