Depends on the day...
Normally it's the 40mm pancake, just because it so wonderfully small, and I like the focal length for random snapshots.
My next most used is the 100mm L macro. I use it any time I think I'll be shooting people. I'd love to have the 135mm or an 85mm for that, but the macro is still very good - and I can shoot macro when I feel like it.
After that is the 24-105mm, that normally comes with me if I think I'll shoot landscape, or just have really no idea what I'll be shooting, but I want to do something serious.