What’s fastest & best way to transfer from old 2019 16” MBP to M1 MBP?

There are different ways supported by Apple. The method that has worked 100% of the time for me (more than 20 such transfers over the years) is to make a Time Machine backup on an external drive and then migrate from that drive to the new Mac upon first boot up on the new Mac. If your external drive is an SSD, it will be fast, a 300 GB migration might take under 30 minutes. External mechanical (HDD) drives are much lower cost but also several times slower, e.g. what takes ~ 30 minutes with an SSD might take a few hours with a conventional (mechanical) drive.