I make a lot of money by beetle hunting. I used to do it religiously, but now I do it occasionally. I also sometimes buy a ton of turnips if I see someone has good prices, but that's more rare.
I'm not one to "farm" beetles or fishes, because if you do it for a long while (hours and hours on-going) you eventually grow tired of it and not want to catch or fish anymore. And being an important part of game, I prefer not to fall in that. But of course, I admit I did it out of necessity quite a few times, like everyone else
However, I prefer to try the turnips market to really make a noticeable profit. Sure is more dangerous, you have to be careful and also lucky, but I like it much more.
I love using the stalk market but I can rarely do it because I am usually not awake during the times Joan sells them and I don't want to time travel, so I usually end up just farming beetles, which I find really relaxing.
The most efficient and least time consuming way in my opinion is of course the Stalk Market Of course it's not something you can do all the time. But there's nothing quite like getting a locker full of turnips and having a field day with a 500+ turnip price. But when I'm not doing turnips, I do enjoy leisurely going to the island and picking up the rare bugs and fish that are around at night I used to exclusively go to the island before I learned of the magic of turnips and the usage of the internet to further increase your chances of selling said turnips
Turnips and Perfect Fruits for me. I get overwhelmed though, I have no consistent method cause I get bored of repetition. So I often just suffer having no bells. Haha.
My favorite way is by storing fossils. I don't need to pay for anything immeditly and I tt a lot so i'll store fossils untill I have a full pocket and sell them all, it's like pulling a slot machine jackpot, it just takes a little time to set up each time lol