Add-ins are the most professional and flexible option because they integrate directly into your slides without requiring you to switch between applications.
If you prefer not to install anything, you can download pre-made slides that already have functional timers built into them. How to add a Timer to PowerPoint slides free download countdown timer for powerpoint
: A highly recommended, modern add-in that supports countdowns, stopwatches, and radial (circular) timers. It works across Windows, Mac, and Web platforms. Add-ins are the most professional and flexible option