Mine is the maxim that every word and clause in a contract is presumed to have meaning, and to have a purpose for being included. Reyes v. Metromedia Software, Inc.
(S.D.N.Y. 2012) (a court must evaluate the disputed language "in the context of the entire agreement .... [to] safeguard against adopting an interpretation that would render any individual provision superfluous.").
As a deal lawyer, this maxim is what makes my job challenging and intellectually rewarding. It serves to remind me that words matter, and that precision and concision are the tools of our trade.
What's your favorite?