2005 Thomas Jefferson
The 2004-05 A-team from Thomas Jefferson High School for Science and Technology, in Alexandria, VA, is widely considered to be the greatest high school team of all time. When playing in tossup/bonus formats with their full lineup, TJ A went undefeated for the entire school year, and is believed to be the only team to do so while playing a significantly competitive schedule (the mid-Atlantic was the most competitive region in high school quizbowl at the time).
The members of TJ '05 used pseudonyms from the movie Top Gun at many tournaments during the year and rotated them across rounds.
Thomas Jefferson won both the NAQT HSNCT and the PACE NSC. Thomas Jefferson's 2005 team remains the only team to go undefeated at both tournaments in a single year, and still holds major scoring records (including: points per 20 tossups heard at HSNCT, points per bonus at HSNCT) at the events.
In 2005, TJ's B team also accomplished a unique feat by reaching the semifinals of the NSC.
This team (joined by Ajay Suresh of Princess Anne) also represented Virginia at the 2005 PAC, and won after Maryland negged on the final question. While not credited to the school, it could be argued that the members of this team in fact won a third national title in 2005.
It is unlikely that any team will ever achieve a similar level of dominance again, since the widespread adoption of effective quizbowl study methods, greater availability of practice material, expansion of interest in highly-competitive quizbowl, and adoption of round-robin playoffs at the NSC all make it more likely that even the most dominant team in the country will incur at least a few losses at some point during the year.