I am a bit surprised nobody mentioned the Stihl Contra yet. Sure there have been a lot of exotic and revolutionary designs throughout history, but the ones that really count are the designs that stayed.
And I can speak only for European market.
Stihl KS and Dolmar CL were the first reliable two man saws that convinced many loggers to go to motorized logging imo. (I was not around then but that's my impression)
Mc Culloch gets my respect for developing the first saw with a membrane carb, a true revolution in the industry. Same for Solo (Rex) bringing the first all position carb with the Rex model.
The Stihl Contra is a real legendary saw to me. One of the saws you could really rely on to get the job done, without tinkering and fiddling all day long to get a saw running properly (flame suit on....). It put Stihl on the lead, an there to stay.
All of the Jonsereds from the '70's are a revolutionary design to me. And good looking on top.
The Stihl 08s deserves its place as well. A dead reliable design that afforded a zillion of homeowners to cut their own wood at a reasonable cost. And versatile as well for other uses.
Husqvarna gets my credit for their effort in the AV design. don't know which model first got Springs for AV, but I enjoy that feature every time I use a saw.
The 500i design may be on its way to get its crown as well...but too early to tell.
And the Stihl "flippy cap" gets my vote for the most useless piece of crap feature ever !!! Sorry for bringing that up again....