island sharks and beetles. I earned most of my money this way. Then, turnips. When I had good prices, I'd open my island and other people would tip me! Otherwise I'd go to other islands to sell.
Turnips were too much trouble for me. Waking up early on Sundays was always too much of a pain, and then I'd invariably forget to check the turnip prices and by the end of the week I just had to sell them for whatever garbage price was available on the last day (I didn't know any tricks about keeping them longer).
Most of my money comes from beetles and sharks on the island, as others have said. I know this topic is very old and the OP is assuredly filthy rich by now... but perhaps some other new person will read this topic and learn some new tricks, like I did with the turnips!
Turnips. I used to buy them often in the beginning and I sold them at other peoples towns.
It was hacked but I think I once sold turnips for like 936 each
Sell fossils on the flea market at Re-Tail. At night, go to the island and catch beetles. I believe you can make 450,000 bells every 30 minutes this way. It does require you chop down all your trees and dig up all flowers and shrubs though.