I suppose that in general terms my favorite is the simple Caesar salad. You can do so much with it. It makes a good starter or accompaniment to a meal or add a protein and it's a meal itself.
Taco salad, defined somewhat loosely, is another favorite. Sometimes I use taco meat while other times I use fajita-style meats. For sauce I usually mix sour cream & salsa, then add cheese, onions, black olives, etc. This is one time I prefer shredded iceberg.
I do like spinach salads too but only with sweet dressings, which I normally try to avoid. I don't eat any sugar when I can help it and I'm starting to suspect that it's best to limit consumption of artificial sweeteners as well. That's a challenge since I really like the flavor of Splenda. Overall I don't have much of a sweet tooth, which is good, but I drink Powerade Zero a couple times a week. A little off topic, I realize, but the long and the short of it is I'm trying to embrace a diet of more whole and natural foods. As I get older I'm starting to realize I can't eat just anything and stay healthy anymore!