best way to transfer data from iMac to new iMac

The fastest, easiest and best way, is to connect your old iMac’s Time Machine backup drive to the new iMac and Migrate from it during the initial setup.

see > Move content to a new Mac - Apple Support

Do not Migrate using your Wi-Fi network, as it is to slow and prone to fail.

Double click on > Move content to a new Mac - Apple Support < and follow the directions in that Apple Support article.

The easiest, most reliable and overall fastest method is to follow the steps in Setup Assistant and Migration Assistant.