I want to build a machine what are the best specs for ArcGIS Pro machine
Best specs can be found here - http://pro.arcgis.com/en/pro-app/get-started/arcgis-pro-system-requirements.htm
Robert's link is the go-to ... but do be sure to check the link on that page
It really needs to be in a font that size (or flashing).
Of course, if you keep that link with you, you could always try out any machine when you head out to buy one at a store.