It's a toss-up for me, so I say either the Whirlpool Surgilator / Super-Surgilator, or the Kenmore Roto-Swirl.
I say this because the Surgilator survived almost completely unchanged from 1952 (maybe earlier?) into 1987 when commercial belt-drive washers were finally discontinued. It began as a metal agitator, moving to bakelite, then into modern plastics. When anything survives that long on the market largely unchanged, it's not because it's bad!
The Roto-Swirl and Super Roto-Swirls in my mind are tied with the Surgilator for the same reason. The early 50s bakelite 'pregnant' Roto-Swirl was reborn into the Super Roto-Swirl in 1963 (one day I'm going to compare the two side by side) and that agitator also survived the transition completely unchanged into modern plastic form. There was even a stint in the early 70s with this agitator in steroid form for the 18-pound machines. The last commercial Kenmores with the Roto-Swirl were made in later 1987.
It seems very fitting that of all the Kenmore agitators that were available (4 designs of the Straight Vane, the Roto-Swirl, Roto-Flex, Vari-Flex, Penta-Vane, Pent-Swirl, and Dual-Action) that the Roto-Swirl was dominant in the early days of the belt-drive design, and closed it out as well.
That's my vote anyway....
Gordon