Water. Over 90% of your body is made of water and water is also where all of the chemical reactions that keep you alive occur.
intestinal fluid
Interstitial fluid is the basic fluid from which all others are derived, including blood, lymph, and all extracellular fluid. Hormones are chemical messengers and are transported by blood, thought they themselves are not considered a body fluid.
sweat i think!
Perhaps it may induce edema, but in general, interstitial fluid is the basic body fluid.
brain
Many things in the human body are basic. Blood, for example, is slightly basic (having a pH of 7.35-7.45). Pancreatic fluid and cerebrospinal fluid are also slightly basic. Bicarbonate is one of the most important bases in the human body because it acts as a buffer, meaning it helps to neutralize acids in the blood. The kidneys either retain or excrete bicarbonate to help control the pH of the blood.
It could be argued that the most basic building block of the body, as in all other things, is the atom. The answer that you are most likely looking for is the protein.
Water
Peritoneal cavity
stomach acid/fluid.
Our stomach is the most acidic area in our body due to gastric juice.