Best is distilled water, since it cannot do harm. After that I usually just scrape with a sharpened piece of metal that is softer than the coin is. I use brass on bronze. I usually leave silver alone, letting professionals clean them. Personally I do not like olive oil, as I find it doesn't do much, and can darken the coin.
SIlver coins will clean up when put on a piece of aluminum foil in water and a teaspoon of baking soda...here are a few other methods I have used.
