Original SAK. While multi-function folding tools weren't new even in the 1800s, the ability of the Swiss to mass produce and mass-market these made them the vanguard of modern multi-tools and Swiss-Army-style knives.
Buck 110, the archetype folding lockback.
Schrade Old Timer 34OT. Not the first Old Timer, but the most popular of all time - in fact the most popular Schrade model of all time. Made by the Imperial-Schrade Corporation continuously for forty years, and still being copied and sold by other knife companies.
Spyderco Endura. Early pioneer of modern "tactical" knives featuring the following characteristics: black plastic handles, one-hand opening action, locking blades, and aggressive styling.
Having a hard time picking out a fifth that would be considered "iconic."