Well, in my case it would be just time consuming having to respec and warp between areas for 10 daggers. But, not time consuming enough to stop me:
Anyway, since plenty of people wanted to know the critical modifiers (totally expected, btw), I have done the research. These daggers tests were done with 200 AR (What a hassle for Thief's Dagger) and with the "assistance" of the Syan Knights in the Undead Crypt (done merely so people have an idea of backstab power at higher levels):
Format - [Name, Backstab Damage, Damage (from a non-counter 1H Light Attack), % difference]
- Dagger - 1030, 58, 1776%
- Parrying Dagger - 750, 33, 2272%*
- Bandit's Knife - 1028, 58, 1772%
- Shadow Dagger - 1034, 59 1753%
- Thief Dagger - 1028, 58, 1772%
- Broken Thief Sword - 179, 58, 309%**
- Manikin Knife - 1030, 58, 1776%
- Royal Dirk - 179, 76, 236%**
- Mytha's Bent Blade - 1028, 58, 1772%
- Black Flamestone Dagger - 179, 58, 309%**
* Thrusting Sword moveset, does not apply AR the same way as Daggers.
** Straight Sword backstab, probably applied the same critical modifier as Straight Swords (309%)
The slight alterations in damage may be due to invisible decimal numbers in a weapons AR that are not displayed (hence why Shadow Dagger does 1 more point of damage than Dagger), so we can conclude that all dagger backstab animations (with the exception of Parrying Dagger) have the same critical modifier. Also, do not take the % values to heart, CRIT modifiers are very difficult to understand.
Later on, I tested Shadow Dagger with 270 AR, the backstab damage was 1536 and the Light Attack damage was 158 (Confirmed, AR still has increasing returns) which led to only a 972% difference in damage. Another time, when I tested with Hollow Soldiers, the damage per backstab was 1428 while the Light Attack was 238, which is a 600% difference. Then, when I tested a broken dagger on the Hollows, I dealt 188 backstab damage and 8 Light Attack damage which was a 2350% difference.
It appears there is a minimal amount of damage that is added into every backstab, on top of the CRIT bonus from your AR, which is why your backstabs are much more effective the less damage you deal to your enemy normally.
Conclusion: If you want to backstab, be a STR build and use the Shadow Dagger, it's made of Awesomeness