In my experience, it’s the first layer. If you can get a perfect first layer to stick and get that perfectly smooth, glossy upper surface, then it’s a fairly good bet that the rest of your print will go down perfectly.
It’s for this reason there are so many different Z-Probes available for 3D Printers, all in the pursuit of getting that first layer just so. But most of these sensors have at least one problem in common: Calibration and Drift