Which sub-specialty of sonography is the best all-around?

It comes down to personality, personal skills, and interests. Vascular is very detailed and tedious, they interrogate the blood vessels of the body and get Doppler waveform signals/take measurements to determine how well the blood is flowing. Cardiac is more similar to vascular because it's more about the measurements and getting accurate numerical data than it is getting good images. General is more diverse because they scan every part of the body but the heart. They rely on good images to get a clear diagnosis. General does includes some vascular studies like carotids, venous legs, etc. I would recommend that you job shadow within a hospital where you can see all 3 specializations and get a good idea of what's involved in each.