This is a neat little infographic on what the ideal temperature is for running a marathon. According to it, 53F seems to be the best. Keep in mind this is a set based on runners at the New York City Marathon. Nonetheless, it gives you some idea. Temperature doesn't affect everyone in the same way. Elite runners are not affected as much as slower runners are. However, what I found most interesting was that the runners between 4:00 and 4:30 hours seem to be the least affected (after the elites). Look at how even the circles are in that diagram for the runners in this segment.
Personally, I've always felt my best running comes when the temperature is between 50 and 55 F. Haven't done any detailed study to narrow it down any further.