* -ness: This suffix turns adjectives into nouns and emphasizes the quality. For example, "bigness" means the state of being big.
* -er: This suffix compares two things, indicating a greater degree of the quality. For example, "bigger" means more big than something else.
* -est: This suffix compares more than two things, indicating the greatest degree of the quality. For example, "biggest" means the most big out of all.
There are also other suffixes that can be used with "big", like -ish (somewhat big) and -ly (in a big way).
So, the best answer depends on the context!