Best alcohol for vanilla extract?

A common question about vanilla extract is: What is the best alcohol for vanilla extract?

Alcohol concentration
A 1995 study showed that 10% more vanillin was extracted at 47.5% ethanol than 95% ethanol [page 16, pdf!].

Alcohol type
Vodka is the alcohol of choice because it has a neutral flavor. Other liquors can be used, but they contribute flavors of their own. Commercial extracts use a neutral flavored grain alcohol (vodka), but you are free to use rum, brandy, gin, whatever.

For more technical details about vanilla and vanilla extract, check out the vanilla info page and the vanilla extraction tutorial.

Update:
Coco&Me just started an extract experiment with different types of alcohol.