It would be a toss up between Medieval 2 - Total War and Silent Hunter IV.
I have all of the Total War games and expansions and also have all of the Silent Hunter games. Silent Service 2 was one of my favorite games years ago and when I found the first Silent Hunter, I was as happy as a pig in shit, and have been playing them ever since.
I don't play many new games, because 99% of them have no replay value, but these two series are ones that I find myself always going back to. Enemy Territory is the only other 'modern' game I play regularly.
I am patiently waiting for a new Total War with even larger armies, the battles are good, but I want to control tens of thousands of men on the field, and have battles that can last for days on end.