The Collaborative International Dictionary of English v.0.48:

Best \Best\, n.
   Utmost; highest endeavor or state; most nearly perfect thing,
   or being, or action; as, to do one's best; to the best of our
   ability.
   [1913 Webster]

   At best, in the utmost degree or extent applicable to the
      case; under the most favorable circumstances; as, life is
      at best very short.

   For best, finally. [Obs.] "Those constitutions . . . are
      now established for best, and not to be mended." --Milton.

   To get the best of, to gain an advantage over, whether
      fairly or unfairly.

   To make the best of.
   (a) To improve to the utmost; to use or dispose of to the
       greatest advantage. "Let there be freedom to carry their
       commodities where they can make the best of them."
       --Bacon.
   (b) To reduce to the least possible inconvenience; as, to
       make the best of ill fortune or a bad bargain.
       [1913 Webster]

WordNet (r) 3.0 (2006):

at best
    adv 1: under the best of conditions; "at best we'll lose only
           the money" [syn: at best, at the best] [ant: at the
           worst, at worst]