I spend countless hours in Medieval 2.
Either alone or together with my brother playing singleplayer and feeling like two great tacticans.
We never played multiplayer, but always singleplayer, as the most fun part of the game was to argue about how we should use each turn, what longterm strategies we could apply or to just view one of those epic movie-like siege battles Medieval 2 ist famous for. When we played together, we always felt about the game, like it was a good movie.
Having extensive mods which add dozens of historical subfactions , Medieval 2 steel also told me more about heraldy of the European middle ages than I would imagined. To this day I cannot go into a castle in Germany, France or Italy, without noticing at least one or two coat of arms of a local nobility.
Even when newer, more extensive entries in the Total War series released, none got me hooked like Medieval 2.