I usually do balsamic vinegar, a bit of Dijon mustard and olive oil. You can mix it up by adding some lemon or raspberry juice to it. My other go to is caesar dressing. My sister made this dressing for a salad (lettuce, goat cheese, candied walnuts, apples and tomatoes) for all of us for Mother's Day that was quite well received (she emailed out the recipe to us as we all liked it so much):
Salad Dressing (Arugula Salad - Ina Garten's Make it Ahead)
1/4 cup Champagne Vinegar
1 teaspoon Dijon mustard
1 teaspoon minced garlic
Kocher Salt & freshly ground black pepper
1/4 cup good olive oil
1/2 cup thinly sliced scallions, white & green parts (3 scallions)
2 tablespoons dried currants
6 cups baby arugula
Whisk the vinegar, mustard, garlic, 1 teaspoon salt, and 1/2 teaspoon pepper together in a small bowl or glass measuring cup. Whisk in the olive oil, stir in the scallions & currents and set aside.
** I used a small shallot instead, she has other dressing recipes, similar to this one where sue uses shallots instead. I also used just 1 clove of garlic (didn't measure) and eyeballed the amount of ground pepper.
As a note, when I had the dressing today, the onion was a bit stronger, so I think the next time I make it, after the initial time we have it, before I put it in the fridge, I may take out the shallots. But it was still good today.