Hi Sue_SZ,
Thank you for reaching Microsoft Community. I understand that you need to know what exactly is needed for you to connect your Surface Pro 5thgen to a portable monitor.
First, the Surface Pro 5thgen uses its Mini DisplayPort for external monitor so if the monitor you have use USB-C connection, you'll need the USB-C to Mini DisplayPort adapter. See: Surface Pro (5th Gen) specs and features - Microsoft Support
As a Microsoft support site though, we aren't able to provide a single recommendation. We can assist with identifying if a particular model is compatible based on the Surface you have, but we work with so many manufacturers around the world that it wouldn't be fair or reasonable for us to say, 'this one is our favorite'. Other community members may be able to speak to their specific preferences and let you know why they feel that way, so we will leave this thread open for their input.
You may also check this page for more details in connecting Surface to a second monitor: Troubleshoot connecting Surface to an external display - Microsoft Support.
Kind regards,
Marrion