It's really hard for me to rank a game as my favorite one. Still, Civ IV is, without a doubt, one of my favorite games, even if I try to restrict the list as much as I can. I really love Civ IV (with BtS+BAT) and consider it to be perfect, for its genre. Other "near perfection" games, imo, would be Baldur's Gate, Kotor, Wizardry 8, XCOM2 (with the Long War 2 mod, otherwise the game suffers from some serious flaws), Nethack (!) and (more recently) Pathfinder:Kingmaker.