ALL of them!

I've been in business just over 5 years now and they have all been important. But I will say that I didn't always have a clause about advance deposits for new clients, and I didn't always have a clause about late payments. I do now and have for a while in both cases - I learned the hard way how important both of those are.
Another clause that is now standard with all contracts is the section that details the client's responsibilities, including noting deliverables needed to complete the work. This clause is always attached to another with a stipulated reasonable time frame. It keeps the client responsible to the progress of their own project.
Hope that helps!