In terms of anachronistic language, I have to admit I love the Falco books. I have no idea if it is authentic ancient Roman slang (and I don't think the author knows herself), but it just sounds right.
There was another thread about books we hated, and I don't even remember the title or the name of the book. It was a police procedural. But I hated every character in it. Unpleasant beyond belief. And yes, you don't want a Mary Sue, but when your characters are so mean, bitter, angry, and twisted that they should be institutionalized, then you have gone too far.