The bags I use the most are 6×18 and 8×20 3mil poly. That covers all of the common things I keep up to a single adult angel. I have some small 3×18 2mil bags that I use for things like one per fish in a pair and then stick both of those into another bag or some plants. On the rare occasion, I have some 12″ wide bags but I don’t use them very often. I double bag everything (except for plants) so for the most part, I end up with 6mil of plastic.
I have thought about trying out breather bags for shipping so I can not take up as much room in a box, but I haven’t gotten around to pulling the trigger on that test yet. For local things, I don’t need to save the extra shipping space so standard poly bags are fine.