Austria Wins the Battle of Koniggratz (Sadowa) in 1866

If it actually won the war, which is a bigger change than it winning the battle, then Prussia's challenge would almost certain end. Germany would stay a loose federation I suspect and a number of states, including Austria are likely to gain Prussian territory.

In the longer range it drastically changes the balance of power in Europe. France will still be seen as the continents premier military power. German industrialisation is still going to occur but may be slower and is likely to be split between assorted nations. Austria is still likely to have the OTL problems of identity and lack of reform and success in the 1866 war will probably make those worse. Also its likely to maintain its presence in Venice at least, meaning continued tension with Italy. This might prompt further French support for the Italians and a defeat for Austria there. Austria will also face growing Russian pressure in the Balkans so its likely to find itself fighting one or more great powers and without significant reforms or allies I can't see it being successful in the longer term.