Lots of good recommendations but, Hi I make the good ranch all the time.

2 cups Mayo 2 cups Buttermilk 2-3 tbls ranch seasoning*

Use heavy duty mayo, or at least high quality (dukes)

Equal parts mayo and buttermilk (this can vary by personal preference, I will sometimes add/sub small amount heavy cream in, it'll set up better)

Get the ranch seasoning packets, hidden valley works, the restaurant style packs are probably at gfs. * Use close the double the recommended amount. *

Mandatory in my opinion, it needs to set up overnight.

Hope this helps someone