I don't expect my chosen celebrity will be a wildly popular answer today. But since I don't know any celebrities personally, it's unfair and ludicrous for me to be picking one as my favourite based purely on conjecture or how they sing or act or look all fixed up and air brushed for a magazine cover. But that's sort of what I'm doing anyway. For the sake of an answer.
So Simon Cowell it is. Why? Because he appears to be brutally honest about absolutely everything. He is not at all stand-offish about voicing his opinions. He says what he thinks. His insults are never purely for the sake of being rude, although he invariably ends up sounding somewhere between cheeky and boorish and looking all surprised when someone takes offense. I get the feeling that, no matter what, he will tell you the truth. Even if you didn't ask and have no desire to hear it. He's the guy who says pretty much exactly what the rest of us are thinking but are too meek and polite to say out loud.
So, love him or hate him, he is who he is. There is nothing fake about Simon.