This one doesn't count as "most decal intensive single piece", but, the decals for it do add up.
Build a 1/350 US Aircraft Carrier, use Starfighters decals for the Flight deck and the air wing's aircraft.
For even more fun, do something like the early Enterprise and air wing in 1/700.
the Carriers look really cool when you finish them, but, we don't get them decaled in a day, lol
note, model carriers are a trap,,,,,,a trap, I tell ya,,,,,you buy the ship kit, then enough extra aircraft to fill out the deck and the hangar deck, then you can buy an add on hangar deck for some (scratch build it for others), photo etch for the ship, and the aircraft if you choose, then update Islands and detail fit for your specific cruise, underwing stores for the aircraft,,,then get authentic paints,,,,,,and a decal sheet for the ship and a separate one for the air wing (if the dates differ),,,,,,,,,,good news, though,,,,,after doing a couple of ships this way, your 1/72 or 1/48 aircraft models will seem HUGE, lol
Rex