Brands to avoid, mainly due to price.
Winix
Levoit
Coway
Everything else is decent.
Don't worry about CADR rating, it's irrelevant, because they're all too loud @ the peak setting.
Just look for 12inch x 12 inch unit, run it on low or medium, 24/7, door closed, 1 per room.
A small oscillating fan in the room helps greatly with filtration efficiency. Point it at the wall diagonally.