A stupid zombie thread, but here goes.
Most powerful standard production: AEA .72 Zeus. Will cost you $1000+ for the gun and likely more than that for the charging equipment. Sure you could do it with a $50 pump, but you’d spend hours pumping.
Most powerful custom builds: 20mm running on helium. If you have to ask the price then you can’t afford it. I’d expect to pay $10,000.
Both will be easily outperformed by a $200 12 gauge.
Of course there are also the pumpkin chunkin air cannons and the air artillery used by ski resorts for avalanche control, so even the ones listed above aren’t the “most powerful”.