Gosh, what a difficult question.
So many agitators that I love. I think it would be better if I answered what's the agitator that I really hate. I'd easily answer the Brastemp 12-vane "blendererator". It's a multifunctional agitator, it can dice, shred, slice, grind, mince, puree and also cut julienne. It removes stains like no other, as there will be no fibers left for the stains to remain attached to. (add to that a miserable rollover)
It's very hard for me to pick a favorite when we're all surrounded by so many awesome designs. I like a lot the Whirlpool's Super Surgilator and it's much smaller brother that came with the Brastemps. I also have a special feeling for the Easy's Spiralator (Which is also my husband's screenname here at AWO and I'd kill for an Easy twin tub washer because of that).
I can't forget the Electrolux DP6 (My design) that equipped the Electrolux LQ11 and LF11 washers. It had a rubber clip on accessory to cover the fins and wash sneakers. There's also the White-Westinghouse agitator that I don't remember if it was available here in the USA or not, but used in Brazil until 1997 (Electrolux-Prosdócimo) It had two parts and the top (also lint filter) was removable to wash delicates.
And of course, the very first we never forget: Bendix Economat, the washer that washed my diapers. That agitator was simple, but the rollover was the very best I've ever seen in my whole life (ok, the rubber "tub" design is what really caused that amazing rollover)
I think, considering the "dramatic" action, I'd stay with the Whirlpool Catalyst agitator as favorite.
In terms of beauty, the new Electrolux "care machine" agitator is my favorite.
Now in terms of classic beauty, it's being a much harder decision.I'm between the Frigidaire 3-ring pulsator, the Philco "wobbling" agitator and the Maytag turquoise agitator.
Now for top performance, the favorite agitator for me isn't an agitator, but a drum baffle that came with the Electrolux Ecoturbo (Frigidaire Affinity). Here in the USA the Affinity was ridicularized with a stupid dumb down water level. In Brazil the same washer used 4x more water per cycle and had full speed spin between the 5 rinses.