Tale of Two Cities is one of my favorites written by any author period. So, I'd have to go with Tale of Two Cities, but I love most all Dickens. I think Great Expectations was the only one I don't much care for.
Hard Times and Oliver Twist probably tie for second best out of those. But, Oliver Twist and Christmas Carol tie for second in general. Hard Times is third when you throw Christmas Carol in there. Don't know how that ranking works, but it does in my mind.