Stealthiest

from The Collaborative International Dictionary of English v.0.48

Stealthy \Stealth"y\ (-[y^]), a. [Compar. {Stealthier}
   (-[i^]*[~e]r); superl. {Stealthiest}.]
   Done by stealth; accomplished clandestinely; unperceived;
   secret; furtive; sly.
   [1913 Webster]

         [Withered murder] with his stealthy pace, . . .
         Moves like a ghost.                      --Shak.
   [1913 Webster]