Ever since I lived in France, I've been a vinaigrette dressing fan. Many Americans don't like vinaigrette - mostly because I think that American-made vinaigrette are too much like us: harsh, wild and unrefined. A classic French vinaigrette is relatively mild and has only a little bite to it. The acid doesn't make you cough or hit your nostrils the way a harsh vinegar does.
IMHO, most commercially available vinaigrette is downright awful so my solution has been to dress my greens in a little extra virgin olive oil - I like a nice, buttery one instead of the more green piquant olive oils - and some wonderful Balsamic vinegar from Trader Joe's tossed up with a little salt and pepper.
However, I recently found this Champagne Caper Vinaigrette from Brianna's - not all stores carry this particular flavor; I found mine at QFC. It is just lovely, particularly on a baby spinach salad with Costco's roasted chicken on top along with some grape tomatoes and slivered almonds.
My overall recommendation, though, no matter what dressing you are using, is to very lightly dress your greens. You'll find that the American habit of slathering on the dressing probably originated with trying to make iceberg lettuce taste like something other than water. If you use baby spinach, mache, or butter lettuce (my favorite), you'll find that these greens taste wonderful if you don't mask their flavors with dressing.