STEALTHIEST
STEALTHY
furtive, sneak, sneaky, stealthy, surreptitious
(adjective) marked by quiet and caution and secrecy; taking pains to avoid being observed; “a furtive manner”; “a sneak attack”; “stealthy footsteps”; “a surreptitious glance at his watch”
Source: WordNet® 3.1
Adjective
stealthiest
superlative form of stealthy: most stealthy
Source: Wiktionary
STEALTHY
Stealth"y, a. [Compar. Stealthier; superl. Stealthiest.]
Definition: Done by stealth; accomplished clandestinely; unperceived;
secret; furtive; sly.
[Withered murder] with his stealthy pace, . . . Moves like a ghost.
Shak.
Source: Webster’s Unabridged Dictionary 1913 Edition