I can't believe it hasn't been mentioned yet... my favorite in terms of smoothness and taste is malt vinegar, it is excellent!
However I do keep apple cider vinegar, white wine, and red wine vinegar on hand too. I also have basalmic but haven't used it in any sauces yet. As I said my favorite is malt vinegar but I use more distilled white vinegar in terms of quantity. Malt vinegar if my favorite but it doesn't necessarily go with every recipe. My last sauce used my jalapeno mash which had a nice amount of lactic acid in it; I used white vinegar there and it balanced nicely.