After 20 years in the business, well, I really think there aren't any great sites. We all have crazy long checklists, so it's always a balancing act between what you need to know from a potential client verses all that you want to know. Plus, there's the industry that you serve that comes into play too! I do University events, so my request forms would seem very specific to my small corner of the industry. These guys have a simple form, but it hits the pertinent areas:
https://globetrottermgmt.com/meeting-request-form------------------------------
Jacqueline C. Ward, CMP
Director, Continuing Legal Education & Special Events, UNC-Chapel Hill
------------------------------