Agree with the other posts that it should be cheap and dry w/o oak. Proabbly a Sauvignon Blanc. Riesling probably doesn't have enough alcohol/acidity to balance the fruit, though you should also be adding brandy which will give a little bit of the alcohol oomph needed. What you are really going for is a simple wine that will provide a good basel and mesh well with the fruit you are adding to it.
You do need to amke sure that it does taste good on its own, bad tasting wine will still mess up the sangria even after adding the fruit. It's like the rule for cooking wine, if you wouldn't drink it, why would you put it in your food.
9:03 here. Good point about Riesling. I hadn't thought of that.
Some Rieslings would probably be OK (the $10 ones I like from Hogue or Chateau St. Michelle would stand up), but a lot of cheap ones probably wouldn't--they can be sweet and flabby. In the cheap sangria wine category, you can probably do better.