My money is on Coca-Cola.
It has been proven to appeal to just about every population in the world, and for most people in most places, it is a rare treat. Some dude in some African village somewhere may not know what an M&M is, but he sure knows what a Coke is, and he’d probably be delighted to have one for free.
Americans would be more blase about getting a Coke for free than most other populations, but there would be loads of takers, especially if the aliens made sure to put outbthe trucks on hot days.
You wouldn’t have the benefit of a tempting odor, but the big red-and-white sign is immensely powerful.
This is one hypothetical that could actually be tested. Pick a top three or five items and a number if strategic locations and let ‘er rip. Park a Coke truck, a pizza truck, and a cinnamon bun truck in Times Square on 3 different days with similar weather and so forth and see what happens.