I like using word templates as they're pretty high grade and professional (decent sized online library too)..
In terms of legal tips..
Basically cover your grounds on a legally valid contract (party names, briefly identify services/products rendered, amounts, if future delivery specify, etc)
In terms of client experience..
Whatever is simple to read and aesthetically pleasing.. plus header (your company or contact info) & footer (maybe with an attempt to lead for future services / offer future discount ?)