I use the “DCF-Empfänger BN 641138″ . It is relatively cheap and many others have reported good results. The connection is pretty simple, connect ground, and +5V to the Arduino and DCF output to a digital pin.
I connected the receiver to Arduino pin 2, because we can assign a flank-driven interrupt to it. It is important to connect the output to a pull-up resistor. Since I saw some spikes in the output signal (more on that later), I also added an additional bypass capacitor to decouple the power supply. Looking at the DCF board I see a capacitor that may already function as a bypass capacitor added, so feel free to leave this part out.
The schematic then looks like this: