I have had good luck with the Heavy Shot- Heavy Metal 3" 1 1/4 oz 2's and 3's These shells are sometimes a little more money than straight steel loads but are much more effective. If you shop around at places like Macks Prairie Wings or Rogers Sporting Goods and use the Heavy Shot rebates you can get these shells for about 150 -160 a flat delivered to your door.
+1 on all of the above. I shoot #4's for early teal then #3's for big duck. South Louisiana here we don't get many "big" ducks. My hevi-metal kills double my usual victims with kent.
Make sure you pattern a few before you dive into a case. I used to shoot a benelli nova that patterned black cloud really good and killed a lot of birds with it. My Beretta A400 hated the stuff and loves hevi-metal.