Best is a tough term. A couple led warm white spot bulbs from Home Depot for $20 total might be the best when comes to cost effectiveness.
However if nutrients are not limiting the most effective light is going to be the strongest, while spectrum matters the PAR and PUR are going to be king and if using similar technology like led, that lights wattage is going to be a big factor in that power. For example a 10w red led light isn’t going to be as effective as a 50w led light even if it’s white since you are putting out 5x the power.
How you grow it is also important. Is your area deep to grow a big ball or more like a Shallow rectangle and you will spread it out more? Something like a kessil h380 has very narrow optics so it can penetrate a deep ball of chaeto with something like 1500 par, but the spread is small. It is a 100w light and if you have a shallower rectangle shape fuge then a 100w panel light will grow that area better but it can’t penetrate as deeply. The same can go for mounting height as that intense kessil can burn chaeto if too close - so the best varied a lot depending on setup.
Realistically though chaeto works so well, if you are not nutrient or element limited (like iron) the lighting doesn’t really matter that much keeping in mind the export needs of the tank. A 50g low bioload tank might be fine witha HD spot, but if you have 250g heavy stock a 100w h380 is going to be better since you have to scale up the export. I myself like about 0.5w led fuge light per gallon - aka h380 ~ 200g decent stocked tank. But I am sure others vary in preferences a lot.
Even a weak light of any common spectrum you see from white to red can keep chaeto healthy, it just wont grow as fast with a weak light. If one has chaeto falling apart or dying (unless it’s a huge ball and shaded), it’s missing something in the water: phosphate, nitrate, iron or other elements, it’s almost never the light that is to blame.