50 for me, but there are plenty of times I would wish for a 35 also. 50 is my general lens though, and 35 is more a specialised lens for me, usually for street and event shots, and when I need something wider.
40 would be ideal for me, and if I ever found a camera that was as suited to the 40, as my M3 is to my 50, it would sorely tempt me. I cannot ever imagine giving up the sublime viewfinder of the M3 though.