Your preferred language's debugger. It really does show in coding interviews who has control over their environment and can debug rapidly. It doesn't matter if

Your preferred language's debugger. It really does show in coding interviews who has control over their environment and can debug rapidly. It doesn't matter if it's rudimentary pdb (basic python debugger) or LLdb, understanding how to structure code such that it's easily testable -and- debuggable is a hallmark of a senior developer.