OK, this has to be a purely subjective answer...and in no specific order:
1: 6200/04/05..... the first official submariner. The birth of what most people understand today to be Rolex. I kknow it originated from teh turn-o-Graph, but the first Submariner has to be in there for me.
2: 6098...explorer is born. Rolex was always a tool watch, but the explorer established it as THE tool watch.
3: 6541.... launch of the Milgauss...with that jagged second hand. Iconic and again showing Rolex to be innovative providers of watches that have real specific purpose.
4: This isnt the first Dayto from Rolex, so not an original. However, the 3346 is the watch that for me really gave birth to that "Rolex" personality. It looks like a modern watch and it was first produced in the mid-1930s. If i saw one of these for sale, wild horses could not stop me!
5: Comex. Just because.