The best information to request is the essential information that you need in order to move users farther into your sales/service pipeline.
By requesting information with that critical function in mind, you can keep your forms short, sweet, and to the point.  Users can become wary if you ask for a ton of unnecessary information, leading to form abandonment and diminishing interest in your product or service.
That’s why so many forms — especially forms at the top of the marketing or sales funnel — only ask for very basic contact information.
By using essential information as your guiding star, you can keep your forms brief while gaining the opportunity to gather additional information later, if required.