If you go the modern approach, then, the 20mm f1.8 and 24mm f1.8 are excellent choices.
If you go the AI or AIs route, then, most of the 20mm and 24mm are quite good. Some can be larger and heavier. Mongo still uses a 20mm f3.5 as the smallest and gives very reasonable results (especially at f8 and f11). Had the 24mm f2 but found it was not great wide open but would perform very well stopped down. In the case of the 24mm f2, if you are not using it for journalism and having to stop down to use it for landscapes (which are generally stationary), then, there is little purpose in f2. Would suggest you try 24mm f2.8 instead.