I will be getting a new iMac. Where is best place to save contacts

iCloud Contacts is the method I use. The contacts automatically sync with iCloud and my Mac, iPhone and iPad after turning on the iCloud Contacts option on all 3 devices. See: Set up iCloud for Contacts on all your devices - Apple Support

Yes, the contacts stored on your Mac in the Contacts application can be synchronized with iCloud, meaning that you’ll see them both on your Mac and iCloud.com. You need to turn on this functionality by going to System settings > [your name] > iCloud and turn on Contacts. You may want to do the same for other types of data, such as Calendar, Notes etc..