Hands down, its the:
STONE DRAKE
*6HD
*40ft, 40 Climb
*+8 Nat Armour
*16 Str, 15 Dex, 19 Con
*Attk: bite (+3+BAB, d8+3+d6acid)
*Full Attk: bite (+3+BAB, d8+3+d6acid and 2 claws (-2+BAB, d6+1)
*Racial +4 Hide/Move Silently checks
*Sleep and Paralysis Immunity