Nothing thick and almost creamy. Something smooth, sharp and crisp.
1 part vinegar to 3 parts oil. Whisk in a tablespoon of dijon mustard as an emulsifying agent. Add chopped herbs such as tarragon.
If you want a sharper vinaigrette make the ratio 2:1 (oil to vinegar), add a dab of mustard (any type of dijon), S&P, and a pinch of sugar.
I have to say, most of the French people I know make their vinaigrette rather creamy. (The more mustard you add, the creamier the dressing will be, and French people love their mustard.) Like HalfPint says, if you want a sharper dressing, elevate the normal ratio of acid to oil.
I think the perfect dressing is: a spoonful of Dijon (good quality and spicy), a crack of pepper, a pinch of sea salt, the juice of 1/2 a lemon and a glug of olive or avocado oil. But here are step-by-step instructions, straight from a French mother's mouth: http://theroamingkitchen.net/french-vinaigrette/
I think of a very basic vinaigrette as about (per your taste) 3:1 olive oil to red or white wine vinegar, Dijon, salt & pepper and a tiny pinch of sugar. Herbs can be added per what you're making, if you like, but not necessarily. Whisk the Dijon with the vinegar, s&p and pinch sugar, then whisk in the oil gradually until emulsified.
Admission: I've abandoned the whisk for vinaigrette. I do the same order of ingredients, but whir it in the blender. Perfectly - lazily - emulsified every time.