At the risk of causing disappointment, I have to break it to you: according to Spanish people I’ve known, there isn’t really any single authentic recipe for Sangria. There also isn’t, to my knowledge, a recipe everyone loves.
There are some basic guidelines that have evolved over the years, but Sangria is really just a form of punch. Like most punches, it’s less about the flavor and nuance than it is about getting people drunk quickly and cheaply. It’s traditionally made from whatever cheap leftover wine and fruit you have that’s about to go bad, and it relies on ingredients like sugar and cinnamon to make it taste good.
By the way, the Sangria is really a party drink, and not really designed to be ordered in a bar. Making up a batch of this punch and sharing it with friends is the best way to enjoy Sangria.