[h00key]

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.

[heart888]

I use Hiew, it's very powerful. Can do initial code analysis, decrypt hex, and even modify hex value.

[eurekaa]

hexworkshop is a good one

[FakeAVHunter]

HXD is the best editor from my life ever