I use several ones. HxD is good for windows. On nix, whatever is easily available.
But dhex (for linux) deserves a special mention as it has diff mode which is very useful for comparing EXEs and other files.
[Poll] What is your favorite hex editor?
Re: [Poll] What is your favorite hex editor?
I use Hiew, it's very powerful. Can do initial code analysis, decrypt hex, and even modify hex value.