The toughest civ in my games is usually the Arceans or Altarians, occasionally Rorians or Drengi. When they're on top, the Arceans just rob everyone until they anger someone who won't give up their money, at which point the galaxy gangs up on them. I always thought of the Altarians as being unimportant because they rarely cause wars; if they're the toughest they just sit around looking cool. Then, in one of my games, the Altarians went over to the dark side... I'll never disrespect the magical talking monkeys again! The Drengin are slow to start, and usually end up cooked if they start near the Torians. Given breathing room, however, they become simply vicious. As for the Torians? Fast to expand, but sometimes they spread themselves too thin.
EDIT: Just had an idea. When saying who's the toughest, add what size galaxy you typically play on. I usually play on huge or gigantic. Perhaps in smaller galaxies, the "stupid" Thalan and Terran AIs at at their prime!
