I'll throw in my usual armchair opinion that most daggers aren't intended for knife-to-knife combat. For that task, something with greater reach and a guard that is designed for protecting against (and possibly trapping) an opponent's blade is better - think "fighting bowie."
Daggers have one purpose only - killin' folks, plain and simple. Folks who probably aren't similarly armed and hopefully aren't aware of the attacker. That's very different than "fighting."
To that end, daggers should have blades 6-7" to reach anything vital, slender blades with two very sharp edges, steel that favors taking a razor-edge easily and is tough rather than one that holds an edge well, and a minimal guard simply to protect the hand from sliding forward.
I think that the Gerber Mk II pretty well embodies those attributes. Applegate-Fairbairns (Boker, Blackjack, etc.) are pretty nice, too. There are others, but most companies high-end companies steer clear of this market, as the dagger has literally no appropriate civilian purpose, unless you enjoy the more "personal" forms of hunting wild game
I've said (controversially, I expect) why daggers aren't for fighting. They certainly aren't for utility. A few folks hold that the second edge may be used once the first gets dull, but I think most of us would agree that one good edge is better thanb two poor ones. I say "poor" because by necessity a dagger will have shallow grinds and a weak cross-section compared to a single-edged knife of the same profile. It will also have a fairly weak tip, and I've said why the steel should not stress edge retention. finally, that second edge seriously limits grips, doesn't allow pressure to be applied with the off-hand, and can be a danger to the user when chopping and performing other heavy tasks.
Yes, they're knives. No, (I say this for the benefit of our seemingly confused law-writers), they're not "dirks."
-Drew