What's the best filter for a betta fish tank?

I agree that strong current isn't good for bettas, but I disagree that you don't need a good filter. I use an internal sponge filter with adjustable outflow and it makes all the difference in water quality - ammonia rises really quickly in a small tank and the betta gets stressed. The ideal is a filter that keeps the water clean without creating that flow that tires the fish out, like the ones from Boyu or even Sobo with flow rate adjustment.

I think you're both right but there's an important detail missing. A good filter really is essential for water quality - ammonia and nitrite will kill your fish quick - but people are right that strong current stresses bettas out way too much. The fish prefers calmer water.

What works well is using a sponge filter, like those air-powered ones or the simpler internal filters, and adjusting the output to be really weak. If you can point the outlet at a wall or plants to better break up the flow, even better. Some filters have valves that control the flow, so it's easy to adjust. The sponge filters out a lot of stuff without creating that chaos of water rushing around that makes the fish neurotic.

It also depends on tank size. If it's 20 liters or more, even a small sponge filter does the job. If it's one of those 5-liter tanks (which I don't recommend, but I know lots of people use them), then you basically can't put in a filter without bothering the betta, so then you're stuck with more frequent water changes. But from my experience, a properly set up low-flow filter changes the whole game for your fish's health.