For lighting, overcast is the best, but usually it requires a high ISO to get a good exposure, sunny is good, but youd need either a fill flash or a reflector to fill in harsh shodws.
All depends on what your shooting, and really depends on what equpment you have.
EDIT: The time depends on what your shooting. If you are shooting a landscape, the golden tones around sunset and sunrise are amazing, but for a portrait, it looks pretty bad.
If your shooting people, the anytime after the golden light is gone in the morning untill they come back in the evening will work. You may need a fill flash at certain points of the day (or depending on where the sun in relation to shooting. You always want to try and get it behind you)
Again, depends on what your shooting.