I can't say that I really liked Tiberian Sun all that much as it felt a little off compared to the other games in the series, though I do remember playing a six-hour game with one of my roommates once, and that was a blast. If you're going to download one of these games for single-player fun, I'd suggest Red Alert, if only to play the awesome Soviet missions with their goofy cut-scenes. But the real fun can be had by playing with a friend. For multi-player action, I'd still suggest Red Alert (the units are more fun to mess around with), but any of the three should suffice. Bookmark the download page for a rainy day. If you can actually get the games up and running on two networked computers, I promise that you won't regret it.