I'm surprised that nobody has brought up honeypot ants.
They are social (unlike most insects), which, given breeding and domestication can be used by humans similarly to the way we use domestic animals inherent socialization instincts (dogs-packs, horses-herds, etc) by substituting humans for the instinctual social group. Honeypot ants are literally walking food containers. I could see these becoming something similar to a milk-cow easily.
Another very obvious choice is bees! We already DO domesticate and use bees in agriculture. There is a very ancient tradition of animal husbandry with bees. Basically, giant bees being used to help humans is a natural choice (once again, notice how we usually co-opt instinctively social animals?) I have no idea how the difficulties of the Square Cube law would apply to a gigantic flying insect, but it is entirely possible that if super sized bees were too weak to fly, they could get to food sources efficiently enough by just running. Perhaps bees would even be bred to minimize the stinger to the point of uselessness and kept as pets! (say, dog sized) Or, perhaps the stinger is retained, and that is the POINT (yuk yuk) and they would take the place of guard dogs.
Grasshoppers (and variants like locusts) are already traditional food sources going back to antiquity. They have more meat on their bodies proportionally than something like a cockroach or beetle, making them more efficient as a meat-animal. They also eat grass, so that's a pretty natural replacement for beef cattle. Once again, they tend to exhibit flocking behavior, which is useful when ranching because it keeps your herd together (one more time: we tend to domesticate social animals historically).
Another insect with a higher proportion of meat over something like a beetle is the potato bug. Putting aside it's utterly hideous appearance, potato bugs would be a pretty good meat animal as well.
Beetles do make sense for work like pulling things, though the solitary instincts and lack of brainpower might be downsides.
One thing to keep in mind: when raising a giant bug for food purposes, you don't need to go through the entire life-cycle. Many insects have a VERY useful "grub" or larval phase where they are pretty much helpless, soft, composed of basically nothing but protein, and mostly immobile. Bug ranchers would probably grow batches of insects to late larval stage, then slaughter them before they became full-grown insects with lots of tough exoskeleton that makes butchering much harder. Some ant species will keep other types of bugs as larva in a similar way, rather like the way we keep cattle.
Finally, don't overlook termites! Giant termites might actually be pretty darn useful. Imagine specially bred huge termites excreting natural concrete to build an overpass or even a high-rise! Obviously, this would take a LOT of careful oversight, control, and "training" (whatever that means for an insect), but there are some interesting possibilities. How about gigantic leaf-cutter ants clearing a path for a road? Obviously, the strength of a termite's saliva (or whatever it is they use to glue sand together) isn't going to increase, so a large building would take steel supports, etc, but natural termites and some ant species will make free-standing towers up to 30 feet tall.
