What is the best windows 11 upgrade assistant from windows 10?

You should know Windows 11 upgrade assistant does not support incompatible PCs. 

This is the most significant limitation. The Upgrade Assistant is a official tool that follows Microsoft's rules.

If your PC does not meet the strict minimum system requirements (like TPM 2.0, a supported 8th-gen or newer CPU, Secure Boot), the Upgrade Assistant will block the upgrade and show you an error message.

While there are unofficial workarounds (like registry edits) to install Windows 11 on unsupported hardware, the Upgrade Assistant will not perform them. You would need to use alternative methods like a clean install from an ISO file.

And Windows 11 upgrade assistant for Windows 10 can not downgrade Windows OS to an old version. The process is one-way.

Once you upgrade to Windows 11, the Upgrade Assistant cannot revert you to Windows 10. You can only go back using the built-in "Go back" option in Windows 11 Settings (which is only available for 10 days after the upgrade) or by performing a clean install of Windows 10, which would erase all your data.