I put (Americans) in the title because we all know the wonder that is ranch dressing is an American thing. It's the source of our freedom.
Unfortunately, many of my fellow freedom loving Americans have rarely, if ever, had real ranch dressing. Most ranch I have had in my life before I made my own was watery. This is not real ranch. Real ranch dressing should almost be something you chew, because it's so thick.
The stuff you get in bottles is not real ranch dressing. It's some made up crap. It has preservatives in it to increase its shelf life and they don't put the things in that decrease its shelf life. Real ranch dressing can't stay out of the fridge for long without going bad. They made that fake crap to push more salad dressing. Blame the big salad mafia.
Buy the ranch dressing packets. The same brand, "Hidden Ranch," is true to their name in that the real ranch packets are hidden in the grocery store.
You need to then buy mayonnaise - don't buy the cheap crap here or the lite mayonnaise. We are Americans dammit, and you deserve to have real ranch. Buy the Hellmann's. Kraft is good too. Don't get store brand. Its not worth it. Save the store brand for when your non American friends visit - they can't handle true freedom anyways.
Then you buy buttermilk, and mix it. 1 cup of buttermilk + 1 cup mayo + 1 packet = ranch. I suggest you keep it in the fridge for a bit before using, as this will thicken it up.
Since making my own, I eat a lot more salads.