The best shooting mode to use is the one that works best for you. I typically shoot in Aperture Priority mode. I choose my f-stop and let the camera choose the shutter speed. That tends to work quite well for me until I find myself in a low light situation. At that point I will probably switch to Shutter Priority.
Although I rarely use the Auto mode on my cameras, I did it this week to see how the results look. (I also shot in both JPG and RAW.) If you shoot in Auto mode and you are achieving results that you’re happy with, by all means continue to do so. I am not unhappy with the Auto mode results and I may try it out again. One day of shooting in Auto don’t necessarily provide a good sample. That said, no matter which mode you choose, it’s always good to try out the others as well.
Shooting in Shutter or Aperture Priority obtain very similar results. When I was shooting exclusively with film cameras I used a semi-automatic mode in which I would choose the aperture and then adjust the shutter speed so that it would line up in the camera to the middle position. Technically that would be like shooting in Manual mode on digital cameras.
Correct exposure is a combination of aperture and shutter speed. Aperture refers to the opening of the lens, the amount of light that is let in. It also determines the depth of field. That is how sharp the picture is in front of and behind the focus point. If the Aperture is wide open (the smallest number f-stop) only the focus point is sharp. On film cameras you can easily see the depth of field on the lens. I wish that was available on my digital cameras.
Shutter speed controls the effect of movement by the subject in the picture. A fast shutter speed (the higher numbers) will have the effect of freezing action. Lower shutter speeds can create blur or can convey movement like moving water.
Many people who take my photo tours or workshops often use the Manual mode on their cameras. Manual tends to be good when you’re taking your time to make a shot. Landscapes, night photography and studio settings are good examples of times when you can use Manual mode.
However, with street photography you need to be quick to respond. So I don’t recommend Manual mode for shooting street photography. It is worth it to try out all the different modes even if you do settle in to one mode that you commonly use.