No one is best. Diuretics are used to treat a variety of conditions from heart failure to kidney disease to electrolyte imbalances to hormonal problems and a host of others. The choice of diuretic depends on the disease being treated and the patient's needs and sensitivities. Some common diuretics include Furosemide and spironolactone. See your doc for more specific info.
Varies by indication. There are many diuretic available for use, most often, of course, to control excess fluid accumulation. Depending on the cause and the urgency, doctors have very mild to very powerful diuretics available that can be administered orally if elective, or intravenously if more critical. Monitoring of the bp, blood volume, electrolytes and other minerals and kidney function are critical for safety.