I love these sort of threads for discovering new products!
I'll start, 'Kiehls clearly corrective brightening and soothing treatment water'
Bought on a whim in the airport a few years ago and now ensure I have a stash underneath the bed at all times. I'm convinced my skin starts to visually deteriorate if I don't use it for even one day! It makes my skin so, so smooth!
What's your 'I must never run out of this'product?