which x64 assembler is better?
 

When patching x64 PE exe or dll, x64 assembler is needed.
For example, "mov rax, rsi" must be modified to "mov rax, 1" .

I used hiew to do this, any better suggestion?

I think Mr.eXoDia's x64 debugger has same ability

Why would you need anything better than HIEW? :)
I mean, HIEW works just fine, doesn't it?