Below setting is the default mode for what I usually use in the initial shot / base case condition.
The rule of thumb is to get the aperture as wide as possible so you can produce more crisp & colourful milky way shot. Since there will be more amount of lights that can be captured by your camera, hence you won’t compensate other adjustments (which will be potentially lowering down your image quality) in order to get the right exposure for the object. In this case, I prefer to have F2.8 lens.
Above shot was taken on F2.8 using Canon 16-35mm lens.Without any post-processing involved, you can witness how rich the stars detail is. It may not be the richest one, but at least I can minimise the ISO adjustment so it won't create a frustating noises in order to get a correct exposure.
Try to maintain your shutter speed to be not more than 20s. The reason surely is because you don’t want to capture a “moving” stars or star trails. Yes that the earth is rotating and an out of focus stars - if noticed - are capable to bother your mind during post-processing session. Sometimes, the star trails do still appear on the corner of the image when you capture it on 25s shutter speed. That’s why if the lights is sufficient, getting a shot in between 15s - 20s would be preferable. This is another important reason to get a wide aperture lens in astrophotography.
Taken on 20s shutter speed, the shot looks perfect on the mid area, until I noticed something's not right at the corner.
Star trails appear!
From my perspective, I think this adjustment will be quiet subjective since this particular will be mainly influenced by your camera series / brand. A more recent / high-end cameras are claimed to have better sensitivity so it can produce a less noisy image on high ISO setup. That’s the reason why I can only give you an advice based on the camera that I am currently using which is Canon 6D. In my case, I usually limit my ISO to be around 5000. With such adjustment, I am quite satisfy with the reasonable noise amount that it produces which is quite minimum. By then, I do not need to face a fierce battle during post-processing to reduce the noise level - which is a bit frustrating.
Taken at F2.8 25s ISO5000, I am very pleased to look at the result as the noise it produced is quiet low.
By then, I can save my energy to concentrate on improving the image quality during post-process rather than to fight the noises.
F:2.8 S: 20s ISO: 5000
By using the above setup, I usually can capture a good details of milky way with a balance exposure and minimum noise level. Sometimes I play with the adjustment just to get some variations that I can consider to add in during post processing mode. However, this is need to be noted, that the above setting is usually work well only to capture the milky way, not the foreground. Depend on the environment that you are currently standing, usually you need to have a different setting so you can produce a balance exposures for the foreground that you aim. This explains why taking several exposures is the most essential part on astrophotography.