For me in the daytime: 800 lumen front strobe and 250 lumen rear strobe (red), aimed towards the horizon. Always. What may or may not momentarily annoy an automobile driver zipping past me may save my life one day. And I can honestly say that when I drive my car and encounter another strobe-equipped cyclist, it's not an annoyance in any shape or form. During the day my pupils are already constricted to adjust for the ambient light - no bike strobe is going to "blind" me.
At night it's a different matter. A steady front light aimed down about 10 meters or so in front of me, and a steady red rear light. Always.