When fielding questions like this, I'll often encourage new freelancers to check out what others in their field are already doing. For example, I did this search for freelancers using the keyword, "accountant" and filtered it to only show those with a Job Success Score of 90% or higher. This means I'm looking at freelancers with some experience in this area of work. Then, you can check out their profiles to see what they've included in their portfolios, giving you some ideas of what you might want to include in your own.
A couple of additional thoughts: