You always want what you can’t have — especially when it comes to beauty products. That’s why we’re getting the inside scoop from your favorite retailers on what’s selling out right now, how to sign up for waitlists, and where to go to find the next best thing. Because if everyone’s buying it, you know it’s got to be good.Almost everything Zoolander said should be taken with a hefty grain of salt — except for one: He nailed it when he said: "Moisture is the essence of wetness, and wetness is the essence of beauty." Hydration is everything.That's why just about every new skin-care product features a host of heavy-duty moisturizing ingredients — from oils and butters to hyaluronic acid and ceramides. And when it comes to choosing a cream, "nourishing" and "gentle" are the two main qualities we look for. Turns out, Birchbox customers feel the same way, because Kiehl's Ultra Facial Cream , $47.50, is one of the retailer's best-selling skin-care items.The cream, which contains squalane, avocado oil, and apricot kernel oil, is seriously thick, but it still manages to sink in quickly without leaving any greasy residue behind. The best part? It never makes us break out, and it soothes our irritated skin, especially in the winter. Clearly, this is a product we have on rotation, but it's not the only rockstar in the category. Ahead, we've got four other killer hydrators to check out.