Which NTP server should I use for my product to update time?
I'd contact ntp.org. They have a pool of public NTP servers with DNS anycast for geographic proximity and are generally able to give good suggestions. 

You might be asked to use a subdomain (e.g. Debian uses "debian.pool.ntp.org") so if a malfunction in your devices causes a traffic storm this can be reflected without affecting the rest of the pool.