Depends on what insects you are asking about.
Sundews and butterworts are great for catching gnats and mosqutoes.If you want gnat control the best plants would be the cape sundew and mexican butterwort.
Venus flytraps are great at catching ants and flies.
Pitcher plants are great at catching flies,spiders,wasps,yellow jackets and any other nectar loving bugs.
- Adrian

For my money, in my collection, Nepenthes miranda puts everyone else to shame. In both the sheer number of bugs caught and the size of bugs category. Ants and fungus gnats hang out in the pot and then venture out to get the big globs of nectar on/in the pitchers and fall in. It's in the thousands for sure. It's also caught wasps, mosquitos, flys, a few of those annoying green stink bugs, and even a cicada that stayed alive buzzing nonstop for three days.
Bob

For fruit flies I would have to say Drosera adelae. Fruit flies tend to go straight for that sundew due to its smell, even bypassing other sundews in the process.

For sheer number and type of insect, I would say my Nepenthes sanguinea and Drosera capensis are neck in neck for now. More beetles, ants, flies, spiders (spiders tend to hang out on the pitchers looking for insects and fall in and drown), and wasps go for the Nepenthes sanguinea and more flies, gnats, moths, craneflies, and mosquitoes get nabbed by the Drosera capansis. For its looks and size, the Drosera capensis is capable of capturing suprisingly large prey items. Even spiders, with their ability to walk on sticky webs, are not completely immune to sundew dew. In the end, they tire and get eaten in many cases. My drosera capensis even outperforms the so called Venus Flytrap by catching more flies over a shorter time period than the Flytrap.

Drosera capensis size for size for sheer gluttony.