I was recently asked this question and it surprised me. The answer, straight out, is that there isn't one.
I'm not going to talk about specific makes or models of lenses, I am going to talk in general terms about focal length. With that in mind, any lens can be used for long exposures. Your lens choice should be decided by your composition and what you want to frame.
For example, if you want to capture a stunning, sweeping seascape, with blurred water and smooth clouds, you would use a nice wide lens, such as a 16mm.
However, you may also want to capture a beautiful waterfall from afar, with lovely milky water flower over. In which case you might use a longer, say 200mm lens so that you can bring it in and fill the frame with that waterfall.
The important thing to remember in order to get that long exposure time is the exposure triangle and how the aperture setting (f number) and the ISO both affect the exposure time. I may do another blog post specifically about that in the future, but for now, bear in mind, the smaller the aperture (higher f number), the less light is allowed onto the sensor so the shutter will need to remain open longer (long exposure) in order to obtain the correct exposure. The lower the ISO, the less sensitive the sensor is to light so more light/slower shutter speed is needed.
That is a very brief explanation and as I say, I may do a fuller blog post on it at a later date but I hope that helps.
Again, in terms of lens choice, go for whatever gives you the composition and framing that you are looking for.