Europa Universalis III is one of the best Strategy games (in fact technically it's an RTS, but plays entirely like a TBS), that had me hooked up one week in a game where it was the only thing that I played, it's awesome because you can start with any nation from any period between 1450-1789 and guide it till the French Revolution, I started with a three-province nation and when the game ended I had beaten the Ottoman Empire, my empire reached through Arabia and had conquered half of Canada. It's an awesome and extremely addictive game, highly recommendable.
I know it doesn't fit the description because it's in real time, but trust me, it feels more like TBS