The aliens had better be careful! If they create counterfeit Coke bottles or dispensers, they’re gonna have to answer to the Coca-Cola company (and it’s lawyers)!
In Australia at the moment truckloads of iced water would do the job.
The most universal and effective bait will be a cell phone charger.
It would have to be individually wrapped something. I think very few people are going to eat unattended, unwrapped food of uncertain origin.
Montreal smoked meat sandwiches.
There is no way to include vegans, raw foodies, paleo dieters, even straight vegetarians, without turning away the omnivore majority. So may as well go with the best.
I can’t believe no one said nachos.
Cheddar and Havarti mac and cheese, with a just a touch of Asiago and spattering of bacon bits.
^ You got Serak, the Preparer working for you?
Fried bacon. It is said to be the smell that will tempt even the most committed vegetarian.
It isn’t food, but cocaine would have the highest retention rate.
Also known as crisps. When I have one, I invariably eat the whole package. Medium sized. Freshly fried they smell good too.
Please put some cold beer in the food truck as well. If possible, German, Chech or Belgian beer.
Sounds very convincing indeed. Must try that with the beer, then the crisps, then maybe more brownies, one more beer… 
You want something that will attract the most people,
you are only allowed ONE type of thing,
and you need to use it all over the world?
Pizza.
Preferably a tasty vegetarian one. (many cultures will refuse meat+dairy mix, many cultures will reject processed sausage like peperoni, but almost everyone will eat basic tomato n cheeze )
It is almost never the #1 best in a population, but it has universal appeal.
I don’t think there has been enough attention to the premise: The OP wants a food that attracts a wide variety of people and in significant numbers. So some sort of bland beans and rice dish would be universally acceptable, but a giveaway would not attract large numbers of people. You can get even more universal appeal with giving away water, but that only attracts large numbers of people if they are thirsty. Fried bacon or pork barbecue would have intrinsically limited diversity. Free ice cream would attract large numbers of people, but primarily people of northern European extraction; that’s not necessarily too bad for diversity, but we should be able to do better.
My guess is that the most universally appealing food item would be some sort of alcohol, so the people who have already said “beer” are on the right track. Wine or a popular cocktail would also be a good choice. This assumes that the aliens are comfortable with a food (alcoholic beverages have calories and can be digested, so they are foods) that would appeal primarily to adults. But people from many different cultures will really turn out for free booze. The aliens could easily disguise the beverages as promotional items from a winery or brewery.
If alcohol doesn’t work (e.g., the aliens insist on including significant numbers of children), then maybe fresh chocolate chip cookies? These seem to appeal to pretty much every culture. You will lose people with gluten sensitivity, but that doesn’t really lost you that much diversity.
You specify “one food”, so beverages are out. Many good answers here, you to consider dietary restrictions so no meat, especially not pork. Too bad, because frying bacon smells … aaaaah. But bring them in with the smell of fresh baked bread. If not that, then something spicy or curried. But vegetarian.
And drinks. Beer, fruit juice, water. That should cover just about everybody.
Whatever bait they put out, it will have to be large and squishy enough to hide the hook. It would be difficult to embed a hook in a Brazil nut, for instance; lots of alien fingers (fingers?) being punctured as any hard bait crumbles. An M&M imitation could be made with the hook already embedded, but anything you might snag on a #18 or #22 hook could be outside the alien’s game rules, or just thrown back as “too small”. Or perhaps the aliens see children as another species, an interesting “side catch”.
A Twinkie with a hula skirt?
And before dismissing good ol’ hook and line techniques, consider how boring the aliens’ job must be, endless light-years of catch, probe, and release, with very scant chance of a trophy. Fishing with M-80s is only exciting the first time, plus it can attract the game warden.
Cheap beer, perhaps with sports-themed labels, would work - see how popular THAT bait is around the that famous fishin’ hole, the Super Bowl. But where to hide the hook? And you’d only snag the one sub-species, drastically skewing the study’s results.
Dan
I dunno about beer or alcohol. Sure, a lot of people worldwide love beer, but is that number higher than people who love chocolate, sodas, or sweet baked things?
Also, beer is a treat that is good to consume only during certain times/situations in order to be enjoyed (I know, it is always 5 o’clock somewhere). A lot of beer lovers might pass up the beer because it is morning and they are going to work or whatever. it is always chocolate time!
Doners and/or fries. Two popular items available all over the world.
Is that a joke? Surely, you know people who will not eat sushi. It’s not universally popular by any means. Personally, I love it. But if trying to think of a food that everyone likes, that would not make the top 100.