A pilot would probably tell you that the most dangerous weather is unexpected weather. If there are bad weather conditions but the pilots are aware of them then they can either use safety guidelines to land the plane, or they can avoid the weather all together. If a pilot is landing on an icy strip without realizing it things can get out of control quite quickly.
Other than unknown weather, the most dangerous weather would be freezing rain. High winds coupled with an icy runway can cause the plane to slide off the side or end of the runway. If the weather is freezing rain it can freeze the flaps and prevent the plane from lowering, and thus landing. It will also freeze the runway quickly. This kind of weather occurs when the temperature is close to zero Celsius. If the weather were to be below freezing there would be no new water to accumulate on the plane and runway, any new snow that fell could be removed. When the weather is close to zero it will rain and then freeze, making it impossible to remove. Freezing rain can also lead to undetectable frozen runways, black ice can be very difficult to detect until you've slipped on it.
