Shogun 2 or Medieval 2 depending on which era you find more interesting. Rome is also great, but quite dated. Here's to Rome 2 being next!
Apart from diplomicy which i think is hardcoded you can get some pretty decent mods to give you a challenge in what ever areas you likeExterminas said:Yes, Medieval 2 or Rome, depending what you like more.
Also: The Total War Games aren't that difficult, so don't be afraid. Compared to the Civilazation Games before the fifth one they are like fingerpaint. Don't even mention something like hearts of iron. ^^