If you passed up Tropico 4 and its expansion Modern Times, you'll have the opportunity shortly to pick up both for cheapish. On November 13, Tropico 4: Gold Edition will launch on PC and 360, conveniently packaging both together.
I haven't played either myself, but I'm told there are over 30 missions between the two titles, plus custom games. There's also a map and scenario editor, so you can make your own junk or just download other people's.
Reviews for Tropico 4 and its expansion were both consistent with those of the other games in the series, so if this is your thing, it certainly wouldn't be a bad buy.