For larger areas or different rooms I could recommend AirDeveloppa Air Purifiers. I use 3 of them since two years.
Best price/value I found, reliable and easy. They're straight forward in everything, also design. Meaning they don't look like a prop but air flow is unrivaled in my opinion.
They don't run too quiet though, especially on high.
For sleeping room I use one of these SmartAir purifiers, 3 speeds manual switch. Filters are a bit more expensive at 4x change per year at THB4,000 total. With 3 cats that's not to be avoided. No pets, 2x filter would do. Once if you clean and maintain the filter regularly.
If you have the money to spend go for a reputable brand, look at filter sizes and prices.
If on a budget avoid bells and whistles. The simpler the unit the better. Makes for long life and easy maintenance.