for me it's the Sims 2 offline, and Kingdom Of Loathing online, where my boyfriend and I have a little clan with our respective best mates from real life. Which is nice, but disturbing when we get together with someone who doesn't play - without noticing it we go off on a tangent about Ascension and Superhuman Cocktailcrafting and Hell Ramen and stat boosts, and then we notice that the non-player is looking extremely confused and we feel very guilty.