I have thought a lot about this question lately since I live in rurality and most of my pictures are landscapes, seascapes or townscapes. I am not sure there really is a good answer to this question, but I think the general advice people often give about using wide angle lenses for landscapes is too simplistic. Wide angle lenses fit more of a landscape into a frame, but on the other hand, they also make the background smaller and more distant-looking than how we perceive our surroundings when out and about. Sometimes this can be used for effect, to make the distance from the foreground to the middle ground and background seem longer than it is in reality.
Which focal length is the best probably depends on what types of landscapes you photograph, but with the landscapes I am currently in in the western parts of Southern Norway where there are many fjords and mountains behind coastal landscapes between valleys, I often find that wide angle lenses just reduce the drama of the mountains behind the fjords by making them smaller and further away. If you want to convey vastness or infinity, then wide angle lenses seem a good choice, but if you want to convey drama and fill the frame with something else than sky in the upper parts of the background, I think longer lenses are actually a better choice. I have taken some nice wide-angle shots, but often find that the better images are the ones I have shot with longer focal lengths.
Maybe making good landscapes photos with wide angle lenses is also a matter of learning how to compose pictures differently depending on the focal length to not end up with bland, boring and low energy pictures every time one uses a wider focal length? Foreground interest is probably more important in wide angle shots of landscapes than in tighter shots since everything else than the foreground looks far away and small. Unless something draws the eye into a wide angle shot, they can feel very empty in comparison to pictures taken with a narrower field of view.
Often 35 mm equivalent seems like a good focal length to get both a bit more into the frame than when using a standard lens and at the same time not make the background too small and distant. It’s a good middle ground between really wide wide angle lenses and the standard lens perspective.
This brings me to the standard lens. Standard lenses are lenses of around 40 – 60 mm full frame focal length (20 – 30 mm on MFT). People say they give you the same perspective as your eyes, more or less. My favourite lens is my Panasonic Leica 25 mm f/1.4 Summilux standard lens (= 50 mm on full frame). It renders a very natural perspective, but the lens is also really nice in colour reproduction and sharpness. I find this the most useful focal length for landscapes since it is possible to fit quite a lot within the frame (of course depending on the distance to the subject(s) which you can change with your feet, a bicycle or car) while at the same time the perspective looks natural.
I actually often perceive the background as larger and closer than when I use a standard lens. When making the composite model of the world that my brain “sees”, I think I focus in on the further away bits and make them slightly larger than how my eyes actually see them while also preserving the foreground at this slightly larger size while maybe shrinking the middle ground a bit. Especially when there are mountains in the background, they often look larger in my head than with a standard lens in comparison to the foreground. Maybe the ancient Egyptians were on to something with their value-based perspective since I actually am quite drawn to mountains and possibly put more emphasis on them when rendering them in my head than other parts of my surroundings. Anyway, to achieve the same kind of drama with my camera as I often experience when I am in a landscape, I would actually need to use telephoto lenses.
Going even more extreme telephoto than the picture above makes the drama even larger, but at the same time, things start to look quite unnatural and exaggerated. If you want to make the background larger and closer looking to create more drama or a nicer background than just negative space, then a longer telephoto lens is a good choice. I experimented a bit with this effect while taking some pictures in Farsund a while back and it really gave me the results I was after, but to anyone that has been there, it looks a bit unnatural.
So, what is the best focal length for landscapes? It depends on what you are trying to do. For how I perceive the world, focal lengths from around 34 mm to 100 mm equivalents (17 to 50 on MFT) look quite natural in their perspective, while shorter and longer lenses makes the image seem a bit strange because the relationship between the background and the foreground just seems a bit too exaggerated. Sometimes, you would like to use an exaggerated or compressed perspective for effect, but maybe not all the time.
So what I tend to do when I go out with my camera is to have my standard lens on or a standard zoom like my 12-32 (= 24 – 64 FF) or 12-60 (= 24 -120 FF) set to 25 mm (= 50 mm FF). If I encounter a landscape that would benefit from more compression to convey the sense of drama in it, I would go for a longer focal length, but if I on the other hand want to make the landscape calmer, less dramatic and vaster, I go for a wider lens.
Of course, this is just my opinion based on my personal taste at the time of writing this, and over time, I might come to other conclusions. I should probably mention that I have yet to try my hand at wide angle lenses wider than 24 mm FF equivalent (12 mm MFT), mainly because I tend to feel that anything beyond 34 mm already starts to get a very wide-angled look that I am not that fond of using except for special purposes. I think unless there really is a need to go wide or long telephoto, a standard lens or a short telephoto lens is usually the best choice for landscape photography.