the fastest way for me would be just go to the island to catch beetles and sharks. thats it. if i do about 2 trips a day i can end up making 500,000 bells in about 90 minutes.
I mainly make large amounts of bells off of beetle/shark hunting on the island ;u;
Since I have a lot of fruit trees in my village, I'll occasionally do a huge harvest if I'm in a pinch, however.
The easiest fastest way for me would be just go to the island to catch beetles and sharks.
Each trip can land about 400-500,000 bells and I can watch TV while I do it. ^^
Just waiting for the spawns takes time. I cleared everything in the middle and just wait for the beetles from the palm trees. Fish occasionally when I see a fish fin. That's it.
like others have said, goldmining the island for sharks/rare fish and for beetles is preferred-- it's lucrative, plus it's so fun and relaxing.
after that, i enjoy making trades at the forums. when people need a particular item in a hurry it usually pays very well; also, selling forum currency for in game bells is fun, its like a race against the clock picking up all those bell bags and running to my side-character's ABD while my visitor is running back and forth to my town ABD and dropping bell bags, it cracks me up, you don't want to be the one holding up the show!
i go to the club LOL and dance for bells. people dont know this but actually the the two Ls in LOL represent mettaton's legs and the O represents the void, fun fact
My favorite method is selling my perfect fruit in other towns. I also love to sell items to other people and hunt for bugs/fish on the island. I also earn interest from the bank because I have a lot of it saved in there.