Off the top of my head, four kits I'd kill for would be:
A 1/24 Cat D11R dozer with full engine and driveline detail.
A 1/24 or 1/25 Cozad detachable gooseneck lowboy with a jeep and a stinger (this is the setup used by Bill Signs to haul D11R dozers)
A 1/24 Cat scraper (pan) with full engine and driveline detail. and last but not least, a totally modern, 53' reefer trailer with spread, air ride, tool boxes, the option of having the whole thing chrome plated.
I have to stop, I'm drooling now!