Depends on the situation.
The advantage of the IV route is that it reverses the INR the most quickly, however this route is associated with rare anaphylactic reactions. Despite this potential side effect, the American College of Chest Physicians recommends the IV route for treatment of serious or life-threatening bleeding secondary to use of vitamin K antagonists such as warfarin, where the advantage of quick reversal outweighs the risk. The risk can be mitigated by administering the medication slowly, not to exceed 1 mg/min.
The IM route is not recommended given potential for hematoma formation.
Given a 24 hour time frame to take effect, the PO route is more efficacious at reversing the INR than SC; and just as good as IV.
Bottom line: Use IV for super quick reversal; if stat reversal is not indicated, go with PO. Onset of action: oral = 6-10 hours, IV = 1-2 hours. Peak effect: oral = 24-48 hours, IV = 12-14 hours.
Source
Valentine, K. MD PhD and Hull, R. MBBS, MSc. "Correcting excess anticoagulation after warfarin" Up to Date. 14 August 2008.
Vitamin k1 (phytonadione): Drug information. Lexi-Comp