Some people place more emphasis on the bang, some on the buck. So answers will differ.
Good midrange (price-wise) shells are Remington Nitro-Steel and Kent. Personally, I use Remington Sportsman for 90% of my waterfowling, Nitro-Steel for the other 10%. Not afraid to try Federal Blue Box (bought a case a month ago at a good price), and have had excellent results in the past with Kent.
All depends on what your budget is.....and the results you've had with the shells you've been using.
Truth of the matter is: the shells you shoot are probably the least
important component of your waterfowling success. But because they get so much advertising hype, and are so easy/cheap to change up, they seem to get a disproportionate amount of attention. Choke tubes will run a very close second.