I started with the beetles, but once I made my first million, I used it to buy 10,000 turnips, which I sold for... around 6 million? I've never looked back since.
I like to Beetle Hunt!
It gets me like 150,000 each night!
Just leave a tree in the middle,left and right of the island and visit it at 5pm or 8pm for the best Beetles and bugs (Sometimes even sharks!)
Good Luck :3
I too paid off my house loan and 13 pwp's with beetles alone (I only have two palms on my island, everything else I cut down and dug out) beetle hunting can get you over 300k in 30 minutes of work.
I recently started with turnips and so I voted both because now I can just do one round of beetle hunting when I need extra bells but for the long haul turnips can make you 5 or 6 million depending.
I haven't even went beetle hunting on my current town, but with my first one I did a little bit and it's way easier and faster than having to wait hours before checking turnip prices. Plus, it's not unpredictable and if you know what to catch, you will earn lots more Bells than with one turnip.
In the previous games I prefer turnips, though. Usually you could make around 15,000 Bells for one, and more if Nook wanted to raise the price.
At first it was just beetle hunting, but I've bought turnips for about three times now, everytime sold them with profit. But my real moneymaker was spike of my turnip prices. Made 8 million just by standing there and letting people sell haha.