Final answer:
Lispro (Humalog) is the insulin with the quickest onset of action, beginning to work within 15 minutes, unlike other slower-acting insulins.
Step-by-step explanation:
The insulin with the quickest onset of action is lispro (Humalog). Lispro is a rapid-acting insulin analog that is designed to mimic the body's natural post-meal insulin spike. It typically begins to work within 15 minutes after injection, peaking at approximately 1 hour, and continues to work for 2 to 4 hours. This contrasts with other types of insulin such as regular (short-acting), lente (intermediate-acting), and ultralente (long-acting), all of which have slower onsets and different durations of action.