There's no question that a voiceover demo is your calling card. Talent agencies and production companies will always ask for one when you make inquiries about being considered for rosters and jobs.
What is equally important is making sure your demo has original content on it. Scripts written by a professional copywriter and tailored to your range and skillset. You don't want to be using free scripts that you find on the Internet. For example, Edge Studios has a free script library. These particular scripts are fun to practice but they are severely outdated and should never be used on a professional demo. In fact, I've seen other coaching companies use those for their own demos. Not good! Talent agents will be able to tell in the first three seconds, if you have an Edge Studio script used on your demo. You don't want to sour a relationship with an agent before it has a chance to begin.
Avoid using copy from REAL national commercials. You don't want to run into legal trouble with PR firms and agents will know immediately that you are not the actual voice of that commercial. It's a quick way to tell an agent you're not trustworthy.
We at UVT Coaching, have a full-time copywriter on staff. Once our students get to the end of a curriculum, we start the demo content writing process. Once the scripts are completed, the content is only used on that particular student's demo and will never be recycled for another student or anyone else. Demos are for showcasing your talent and should always include original scripts.