Bots/Storm mastermind works for me 
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
Energy/Storm slotted with KB sets maybe. Dirt cheap sets with nice bonuses.
Then again, a lot of sets, like Battle Axe or Super Strength could be KB kings with some KB enhancers slotted. Only 3 (?) or BA's attacks don't do KD, so you could be batting enemies left and right if you wanted.
But for blasters it's likely going to be Energy/. Electricity with Thunder Strike and Lightning Clap could be a contender.
They ALL float down here. When you're down here with us, you'll float too!
@Starflier
I'd say a Storm/Energy Defender would have the most knockback, simply because Defenders get the strongest AT modifier to KB. FF/Energy might also work... you'd lose the chaos of having tornadoes blunder around tossing things up in the air but you'd get Repulsion Field and Repulsion Bomb for AoE KB, plus you get the absurdly high magnitude knockback of Force Bolt. Force Bolt is stronger than Power Push or Power Thrust, is ranged, recharges very fast, and tops out at magnitude fifty KB fully slotted.
Cascade, level 50 Blaster (NRG/NRG since before it was cool)
Mechmeister, level 50 Bots / Traps MM
FAR too many non-50 alts to name
[u]Arcs[u]
The Scavenger Hunt: 187076
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I want a toon that gives me the most possible KB, do I need to pick a energy/mental or is there something else?