User blog comment:Olly of Duke/Dark Souls 2 Parrying Questions/@comment-4511886-20151019141153

The Scimitar's, and most other Curved Swords', active parry frames are slightly delayed. Basically, you have to parry earlier than you'd do with a shield or Thrusting Sword for the parry to succeed.

Upgrading the Parrying Dagger has no effect on its parry. However,  it can be a suprisingly good weapon due to its unique moveset (for a Dagger).

The Mail Breaker is supposedly the best crit weapon, as its crit multiplier has remained untouched, unlike the Daggers. Other than that, the Manikin Knife is probably your best bet.

As far as I've seen, infusions have little to no effect on criticals aside from the Damage Type difference, which depends on the opponent's defence against Physical and Elemental damage.