Consumer Reports testers look at features, noise levels, and how easy a humidifier is to clean. Tests have found bacteria that can grow in every type of humidifier, but some are better than others at keeping them out of the air. Evaporative-type humidifiers are less likely to put bacteria back into the air. They may grow in the tank, but they're not going to come out.
Consumer Reports says the Well at Walgreen's brand evaporative humidifier (model # LEV 320) is a good choice for a medium-sized room and costs just 32-dollars. But keep in mind evaporative humidifiers are noisier because of the fan. If you'd prefer a quieter humidifier, consider an ultrasonic. A CVS brand (model # GUL540V1) is top-rated for small rooms and costs just 30-dollars.
No matter which type of humidifier you choose, the safest approach is to empty, rinse, and dry it out every day. And once per week follow the manufacturer's instructions on how to disinfect the tank.