I am contemplating a watermelon sangria. Have you made one? What kind of wine and liquor did you use? What sounds good?

Depends on what you want.
If it's to use up watermelon on hand, use a classic sangria recipe and go for it.
If you want watermelon flavor, go for another punch recipe because the strong flavors of wine (red or white), brandy and/or orange liqueur will drown it out.
For example this one (like a Cosmo, but with watermelon):
http://www.foodnetwork.com/recipes/geoffrey-zakarian/watermelon-cosmo-punch.html

I would use nice crisp white, like a Pinot Grigio or Sauvingnon Blanc, I usually add triple sec or Cointreau and Chambord. If you want a stronger watermelon flavor they sell a watermelon pucker . Whatever you do add the ingredients sparingly and taste as you go till you get the flavor profile you want. Let us know how it comes out.