First question: fixed or folding?
Second question: blade length?
Third question: how much trouble are you going to get into for having a dagger?
'Round my part of the country, it can be a lot!
I did some research into "fighting knives", and if you're posting here, I'm sure you're aware of the messy medical, and legal, realities that a "knife fight" can lead to. But it's worth bringing up, because the consequences can be so serious.
From what I understand, if somebody jumps you with a weapon, and you pull one in turn, if he takes so much as one step back, and you take one step forward, you become the assailant, because, legally, he is running away! Most felons aren't going to be thinking about that, but as a private citizen, you have to be, and personally, in the middle of such a situation, I don't trust myself to analyze the possible legal fallout from my every word and action.
Which is why I don't carry any knife that I can't make a damn good case for being a tool, not a weapon.
Having said that, see what you think of the Hideaway Knife. Pricey, but hard to beat for retention. Shivworks seems to specialize in nasty little knives for nasty situations, too, and I think Blackhawk makes a few small blades that might fit your needs.