Over many years of entertaining friends and family one of the most consistent pieces of feedback I've received has been praise for salads and requests for dressing recipes.
The privilege of time spent in France and Italy years ago hammered home the decisive importance of using only the very best ingredients and keeping things simple. Today knowledge of many of those ingredients seems to be almost lost, even among many who consider themselves "foodies." Much of the problem (and I'm writing entirely from a U.S.-centric perspective here) is that the best oils and vinegars are almost unavailable at retail in the U.S. - and will certainly not be found in the places you'd think (by virtue of prices charged) would have them, such as the major natural foods supermarket chains.
Olive & Walnut Oils
For the uninitiated, a thorough perusal of the Truth in Olive Oil site is well worth your time. If you want to spend the money to experience at least a few of the oils that set the standards there's no better place to find them than Zingerman's in Ann Arbor, Michigan. You can buy the Romate vinegar (see below) and many other treats there as well. Giuliana Imports in Colorado, in addition to its excellent wines offers a carefully curated selection of organic olive oils that, while not inexpensive, are fresh and offer superb rapport quailty:price. A third option is Tuscan oils specialist The Rare Wine Company.
For value and freshness, not to mention supreme affordability and value, the go-to choice is the authentic Tuscan vintage dated oil from Costco. At around $12 for a liter it's less than one-third the price of top small-producer oils, is 80-90% as good, and will certainly be far better and fresher than any famous oil you find at fancy grocery stores, in part because the latter oils are almost certain to be at least a year or two past date. So while it's good to taste the very best Italian and French oils, you can do a lot worse that to do what I do and just lay in a year's supply of the Costco Toscano (not to be confused with the only-for-cooking generic Costco oil in the huge plastic bottles) when it comes out in the spring and use it for both salads and sauteing.
The only other oil I recommend keeping on hand for salads is a rich, traditionally-made walnut oil, and here, too, you have a choice between the absolute best product, from French producer J. Leblanc, and a much cheaper oil that's 80-90% as good and requires no special search.
The Leblanc oils (the walnut being his most famous product, but his hazelnut oil is also exquisite) can be found on Amazon and occasionally at good retailers. They are nothing short of sublime, and make other, more readily-available oils taste so bland by comparison that you quickly realize they're not worth the trouble.
Second best is a phenomenal American-made oil from Black walnuts that can be found - of all places - at Wal Mart - and for about a third the price of the Leblanc. It's called Hammon's Black Walnut Oil, and the company has an excellent, informative web site.
I owe Ari Weinzweig from Zingerman's so many debts for his pioneering work and brilliance that I wouldn't know where to start with thank-you's, but high on the list would be much gratitude for him pointing out that the single most useful and versatile vinegar to have on hand is a sherry vinegar - not a red wine one, and certainly not the industrial/commercial balsamics that in all honesty should never find their way past your front door for any purpose.
There are a number of excellent Sherry vinegars available, but my favorite for consistency and value is the one from Sanchez Romate (who among other things are makers of one of the two best Spanish brandies, the legendary Cardenal Mendoza). It's a full 750 ml. wine bottle's worth, at a great price from Zingerman's, KL wines and a few others. Plus the bottle art is beyond cool!
Sherry vinegar in pantry, the next task is to obtain a truly great red wine vinegar, and here there is one product that towers above all others - the New Orléans method products from the legendary Martin Pouret. Here's an article on this amazing company from the New York Times. Fair warning though: once you taste the Pouret products you'll never be happy with other vinegars. You can find the full range (along with many other temptations, including hard-to-find Nyons olives, Tarbais beans and duck fat for the best roasted potatoes) at the estimable New York mail order firm French Feast.
At least once in your life you should splurge and spend $100 on a 100 milliliter bottle of (that's actually the least you can spend) of the real thing from Modena, along with reading the descriptions of how the product is made and the various grades in Lynne Rossetto Kasper's magnificent cookbook The Splendid Table, in which you'll learn that real balsamic vinegar was never intended to be a commercial product, and that the substitutes for the real thing we commonly find in the U.S. are unfit for just about any use.
Here again Zingerman's deserves lots of credit (and your patronage) for being the pioneer in educating us while bringing in the best of the best for decades (and if you live anywhere near Ann Arbor you should simply visit, taste as many oils and vinegars as you can, and spend).
For everyday use perhaps the best choice is the Vecchio Dispensa 8 year old, or for a bit more money you may be able to find the Villa Manodori at a local market or online. While far less concentrated than authentic traditizionale these "balsamic condiments" are the least you can spend for a product worthy of drizzling on (and transforming) a simple rotisserie chicken, local strawberries in season, or dressing a Blacksmith's Salad (raw radiccio dressed with a 2:1 olive oil/balsamico blend with slivers of Reggiano Parmigiano on top.
You'll never see a salad dressed with a "balsamic vinaigrette" in Italy. A few drops of one of the everyday-priced vinegars mentioned above, when added to great red wine vinegar like the Pouret, add a lovely sweet-sour note to dressings for bitter greens.
Ratios and Recipes
While Marcella Hazan and many other authorities on Italian cuisine like to dress their salads to taste in a bowl, I find French precision helpful in achieving consistency.
Insalata means "to salt," and without good salt it's not a salad. Any good fine-grain, non-iodized sea salt will do, though the Sel Guérande grey salt from French Feast is especially good (and very easy to order alongside your Pouret vinegars).
For a subsantial salad for two people, start with a generous 1/8th teaspoon of salt. Always dissolve the salt in the vinegar (it won't dissolve in oil) - and of course the same order applies when using lemon juice in lieu of vinegar. Somewhere between three-quarters of a teaspoon and a full one should be just about right for the vinegar, and the starting ratio for vinegar to oil is always 1:3. With experience it becomes second nature to back off on the vinegar for mild greens like Bibb lettuce, or to add an extra jolt for Spinach or Frisée avec lardons.
Perhaps more important than ratios is restraint. The goal is to just barely coat the salad, with no dressing whatsoever pooled in the bottom of the salad bowl. Mixing thoroughly is just as important: I make sure to toss at least 35 times before tasting for balance, thereby only correcting with more salt, vinegar or oil when absolutely necessary.
Probably 80% of the time I make the simplest of vinaigrettes - or what Marcella Hazan simply called "real Italian dressing:" olive oil, vinegar (or lemon juice), and salt. With a little (but not much) more effort one can make an equally classic French vinaigrette by adding half a clove of minced garlic and a quarter teaspoon each of dried tarragon and sharp Dijon mustard (Amora from French Feast is superb) to your vinegar and whisking these with the oil (olive or walnut work equally well).