Noach

THESE ARE THE OFFSPRING OF NOACH; NOACH WAS A PURE RIGHTEOUS MAN IN HIS GENERATION.  NOACH WALKED WITH G-D (6:9)

RASHI- In his generation: some Sages explain this to his credit- all the more so had he lived in a righteous generation he would have been even more righteous.  Other Sages explain it to his discredit- according to his generation he was righteous but had he lived in Avraham's generation he would have been insignificant.

Why would some Sages explain the verse to Noach's discredit?

Noach failed to improve anyone else in his generation, though according to our Sages he did offer rebuke.   This was a failing on his part.  Successful rebuke requires making the effort to understand the root of the problem and mending it accordingly.  Noach did not understand what made his generation tick, and his words fell on deaf ears.  To improve society one needs to work hard to understand it well (Rav Yaakov Kamenetsky).