I can't have the dairy (EXTREMELY lactose intolerant), but my mom has fallen in love the Marie Calendar's yogurt dressings. They are low in sugar and fat. The perfect combo for us SB girls.
I like having a good balsamic vinegar with some EVOO. There's also red wine vinegar, white balsamic vinegar, white wine vinegar, and several others. Try a few, but make sure they aren't the cheapest ones in the store because they will taste nasty especially if you are pouring it on plain.
There's also recipes out there for the olive oil and vinegar shake-it-up dressings. I really like these, but I suppose it's all preference. (My fiance, for example, doesn't like the vinegar dressings).
As for the ranch, I use a little of a ranch dip dry mix with ff plain greek yogurt. (I found a kosher, dairy free version in the international Jewish section. I jumped up and down!! haha) This one only has 2 grams of sugar for a tablespoon, but you only need one teaspoon to make a serving for one salad.