WordNet (r) 3.0 (2006):

top dog
    n 1: a person who is in charge; "the head of the whole
         operation" [syn: head, chief, top dog]

Moby Thesaurus II by Grady Ward, 1.0:

60 Moby Thesaurus words for "top dog":
   A per se, ace, boss, champ, champion, chief, commander,
   conquering hero, conqueror, conquistador, dean, defeater, doyen,
   doyenne, easy winner, electronics king, fugleman, genius, head,
   hero, high priest, higher-up, honcho, important person, king,
   kingfish, kingpin, laureate, lead, leader, leading light, luminary,
   master, master spirit, nonpareil, pancratiast, paragon, paramount,
   personage, prima donna, principal, prodigy, ruler, runner-up,
   senior, shoo-in, star, subduer, subjugator, superior, superman,
   superstar, sure winner, the greatest, the most, triumpher,
   vanquisher, victor, virtuoso, winner