Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter
Is this adapter the best way to transfer data from my early 2015 Macbook Pro (TB2) to late 2020 Macbook Air (m1)?
MacBook Air
Depends on what you mean by "best".
Wireless is easiest and in my opinion easy and free is best. There is a lot of "overhead" involved in the data transfer so a potentially slower wireless speed will not be the limiting factor. Get an adapter only if your wireless environment is hopelessly clogged—which you probably would have noticed already.