Other - use system which seems to fit the game being played.
D20 for D&D at the moment as its a reasonable system which has a simple basis and my group has players used to different versions of D&D rules so best to standardise on the current set. Probably going to use some D20 modern for a little while and the Conan rules from Mongoose which are also D20.
BRP for Call of Cthulhu as it just fits/I'm used to it! Possibly going to do an Arthurian campaign using Pendragon for the good fit of rules to setting.