The best ISO setting depends entirely upon your particular shooting conditions and the amount of available light that you have to work with. Basically, the lower the ISO value, the less electronic noise is introduced, but the 5D Mark II has pretty clean gain all the way up into higher ISO values. Try starting with ISO 100 and work your way up from there to whatever value provides the best image based on the amount of light you have.

In normal lighting situations, I have been treating ISO as the exposure variant. This is because I want a standard 1/60 shutter speed, and want control of depth of field.

In bright daylight, this also requires use of an ND filter. I have ordered a variable ND filter for testing for those purposes, though and ND8 worked well for me in bright outdoor conditions.

This is my modus operandi too. Set the shutter speed (here in 50Hz electricity land that's 1/50 under artificial lights) then the aperture for the DOF required & let the ISO float. The ISO adjusts in a very intelligent way for shooting video as it is 'damped' & doesn't flip up & down all the time with changing light conditions but smoothly & slowly adjusts itself. This avoids the classic amateurish 'pumping' up & down of light levels that is usually seen with auto-ISO even on professional camcorders e.g. Canon XH-A1. You can keep the variable ND filter on all the time except in very low light levels as there is minimal grain even at high ISO. I have switched on Highlight Tone Priority which has a minimum ISO of 200 which negates the effect of the ND filter at minimum.