One you play long enough and know how to control.;)
That's the most correct response I've ever seen lol. But basically it's true. The best dagger class is the one you can play most effectively. Each person has their own playstyle and each playstyle can be suited to each dagger class. It's really what you feel most comfortable with.
I haven't been playing for 2 years (last time it was on Luna) and afaik right now the big problem for dagger classes is PVP scripts and automated PVP. Some antibackstab system that makes backstab pretty much useless. But that's what I've heard, not my own experience