what is the proper bootcamp software for windows 10 on macbook pro early 2011
You should be using Boot Camp Support Software 5.1.5621. You can download the software from the given link. With later versions of macOS, the Boot Camp Assistant can be used to download the software.

You probably will need to install Windows to legacy BIOS boot. This means the drive will have to be hybrid partitioned. This is where Windows uses the MBR partition table and macOS uses the GUID partition table (GPT). Also, you should use the optical (DVD) drive to install Windows. If you have latest firmware installed, then you might be able to use a flash drive to install the latest Windows 10. Currently, with would be the October 2018 update. You can get the latest firmware by having the latest version of High Sierra installed.

You can not BIOS boot from a flash drive. If you succeed in booting the Windows installer from a flash drive, then you will have used an EFI boot method. However, even though you have EFI booted the installer, you still will need to install Windows 10 to BIOS boot. This is because the Boot Camp Support Software 5.1.5621 probably contains drivers that require Windows to BIOS boot. Instructions for properly use a flash drive to install Windows 10 can be found at the accepted answer to the question: How to install Windows 10 into a 2011 iMac without using the Boot Camp Assistant, an optical (DVD) drive or third party tools?