I hate flat sprints. I'd much rather go into a slight uphill or a real uphill.
Basically what rkwaki said is right - if the lead out is good and is going 38-40 mph, a 12 is about the easiest gear to jump in.
When things are not quite so good, speeds more like 35-36 mph, a 14 might do it, maybe a 13.
If there's a hairpin before the sprint maybe a 15 or 14. This also depends on wheel weight, since significantly lighter wheels will spin up quicker (maybe one or two fewer downstrokes).
I did a bit of semi-logical analysis of my sprint. I found that my best peak power came when I jumped at about 100-110 rpm. I think. I have to check my notes. Anyway whatever the rpm, it was at a higher rpm than where I normally jump (70-80 rpm). I consciously changed my sprint one week, actually checking my cadence on my computer about 200m to go, shifted into an easier gear to get into the right rpm range then sprinted.
I found my experiment:
http://sprinterdellacasa.blogspot.co...g-my-jump.html
So I guess I jump best at 95 rpm, give or take, and really taper at 105 rpm.
What that means is the answer to the original question is "whatever gear I need to be in, at the speed when I jump, to be pedaling at about 95 rpm."