Mostly epic. I need to be able to immerse myself into the world to be able to truly enjoy Civ, and that just doesn't happen on quick. Standard isn't bad for peaceful games, but otherwise I like to actually use the different units and not feel like everything is obsolete 20 turns after it's built. Not to mention ancient-classical era civs are actually something to be feared on slower speeds.
Marathon though.. don't get me started. I tried it once and quit out of sheer boredom after 100 turns netted about 3 techs and 4 builds. Much too slow to hold my interest.