If the crown on my tooth came off, what's the best adhesive i can buy to hold it in place?

See the dentist: I assume you are talking about what can hold it in until you see the dentist, you shouldn't be doing it yourself. As a temporary measure ( and i mean a few days) you can clean out the crown and place a gel denture adhesive, toothpaste or they sell some over the counter temp material, but that is usually to fill holes. All these options will not last very long due to the weight of the crown.

Vaseline: If the crown needs to be bonded on, eugenol which is in most otc preparation will interfere with the bond. Vaseline usually is strong enough to hold the crown on until you can see your dentist. See your dentist as soon as you can.

Phone Call: There is no adhesive or cement that you can purchase that will rival the materials that your dentist has. Why did the crown come off? Is the tooth decayed? Does the crown fit well? Do you need a new crown? These are all answers that you will get when you visit your dentist. Please make the call...