Depends on who is available to you. If you're doing all the drawing yourself, then one of the above softwares is the way to go. If you have a piping department, the easiest way is to hand-sketch something and let them draw it. Then mark it up/correct it as you iterate towards perfection.
On the other hand, some simulation softwares will allow you to make an excellent PFD. Depends on the package. I can personally vouch for ProMax (bre.com) being one that lends itself towards doing this activity. If you are designing a process, and creating a simulation to aid in that design, the software package might be able to produce an excellent first pass, which you can then have CAD'd up or something.
Onwards,
Matt