Late evening, between 8 and 11. This is the time where I'm usually done with all of the work that I was planning on finishing and I've usually still got enough energy during this time to do whatever I need to. It's essentially the time of day where everything winds to a halt and people begin to relax.
My second favorite would have to be the hours after that. I usually don't have to fight sleep until around 1:30, so between 11 and then, when everyone's going to sleep, I've got my own little guaranteed-to-be-uninterrupted time to do whatever.