Medieval: Total War
The original Shogun laid a good foundation and subsequent titles/expansions have done the TW franchise justice over the years.
But after 10 years the 2nd installment in the series still holds my attention. Replayability in any game is a must and MTW has it. Eras, Glorious Achievements, Viking Invasion expansion pack, personal mods aplenty, etc.
This is one game that has a permanent place on all my hard drives.

Good to see another MTW fan. I definitely agree about the replayability. I've put thousands of hours into that game... compared to hundreds I put into Rome / Medieval II, and the thirty-forty odd I put into Empire / Shogun 2.
It's sad really; when you mention Medieval: Total War to a lot of Total War fans, they immediately think of Medieval II... even though it's vastly inferior in my opinion.