The "best" block heater is one which warms the oil (like the two in my air-cooled Deutz, which warms the entire drivetrain front to back when plugged in overnight). The magnetic ones barely do that, so as a second option the coolant warming probe works pretty well. It has recently been about 30-40 below here. I have a block heater in my DK90. It warms the engine only, so I had a magnetic one laying around for years which I stuck on the transmission. It works just a little bit, and it did eliminate the hydraulic whine on startup.
This "plugging in for an hour" stuff is a little ridiculous in cold weather. Just plug it in overnight. If the machine is exposed to the elements, even a coolant heater will barely warm the engine.