Most skaters find the toe loop the easiest jump, with the Salchow right behind. The CoP agrees. The triple toe is the lowest valued triple (4.0) , with the Salchow next (4.5). The Zayak rule was put in place because Elaine Zayak could do triple toes all night long.
But in figure skating history, it seems that the edge jumps came first, and it took longer to master the toe jumps, even the toe loop.
First double jump by a woman: Cecelia College, 1936 (Salchow) (I think Gillis Grafstrom might have been the first man to do a double Sal, in 1920.)
First triple jump: Dick Button, 1952 (loop!)
First triple Salchow: Men, Ronnie Robertson, 1955; Ladies, Petra Burke, 1962.
First triple toe loop: Thomas Litz, 1964
How did it happen that Dick Button could do a jump that is still pretty hard for today's skaters, a triple loop, more than a decade before anyone could do a triple toe?
Quads were the other way around. Kurt Browning was credited with the first quad toe in 1988, and others landed them with only the slighest of two-foot landings at about the same time. In wasn't until a decade later, 1998, that Tim Goebel landed the first quad Salchow.