1 part balsamic vinegar, 1 part red wine vinegar, 10 parts olive oil, 1-2 parts dijon mustard, salt to taste (more than you might think. maybe a full part?)
or apple cider vinegar, sweetner, olive oil, salt, and a touch of mustard. (sweetner can be molasses, honey, maple syrup, palm sugar, etc)
you can add garlic to either one, fresh herbs, and other "exotics" (probably only one, and usually to apple cider vinegar not the balsamic). pomagranate molasses, umeboshi plum vinegar, lemon and cultured dairy, soy sauce, all sorts of things. usually only one thing per dressing though. Its better to keep it simple.
the key is proper salting. start by adding a touch, tasting it, adding more, tasting, adding more, tasting, til you learn the spectrum. truely the salt is key. eventually you'll pretty much know how much.