There are many Mandarin frequency lists which show the relative frequency of Chinese characters. For example, the Chinese Text Computing web page has several lists of Chinese characters compiled from various sources.
However, Chinese words are often composed of more than one character, so lists of single characters can be deceiving.
Here is a list of the most common Mandarin words, as opposed to individual characters. Some of the entries are individual characters, but the majority are character compounds which form Mandarin words. Adapted from the Steering Committee of the Test of Proficiency - Huayu
Meaning: Interjection showing surprise, doubt, approval, or consent. Can be pronounced in any of the four tones.
Meaning: modal particle indicating polite suggestion; ...right?; ...OK?
Meaning: (a measure word); (marker for direct-object); to hold; to contain; to grasp; to take hold of
Meaning: white; snowy; empty; blank; bright; clear; plain; pure; gratuitous