That depends on what you do with it. If it is only used to transfer files, a minimum of 8G USB flash drive is enough to hold pictures and documents, and cannot transfer large files.

If a 64G USB flash drive is enough to transfer a game or other things with tens of GB, 64G has the highest cost performance, while 128G is too large to be necessary, and 64G is enough at most. Of course, if you have more money, you can go directly to the biggest one.