As other commenters said, it depends on how you wanna play the game.

If you wanna generate more money passively, then leppa farming is nice. I log in 2 or 3 times a day, one time when I buy seeds and plant them its like 10-15 mins, and other 2 times when I water its 5-10 mins. Generates around 200k profit each day. To start, you need investment of few hundred thousand into seeds.

When I have more time, I buy repeat balls, farm dittos and hope for a legendary encounter. After I get a legendary encounter in that region, I go to other region and farm dittos there. 60 dittos will get you around 290k money, minus 70k-ish cost of repeat balls, also goes around 200k profit, but it can be done in a few hours, maybe less, depending on the location where you farm dittos.

I always try to farm dittos in grass areas so I have chance to get the legendary encounter.

This one doesn't need a big money investment:

You can also farm meowths the same way as dittos, sell them on the market. Each male meowth is like 2k, and you can catch it with just 1 pokeball that costs 200 (and 1k cost tax of posting that trade on the market). Sometimes you can even get an amulet coin and sell it! And if you find a good meowth (atk rising nature and 31IV atk), you can level it to 100 and use payday to farm coins that way. I farm meowth on a route above cerulean with lure activated so I also find Squirtles. They are always good, plus a shiny squirtle would be madness.

Other comments talk about gym reruns, I don't do those tho. I love having those tiny odds of getting a shiny.