Thel 'Vadam, while initially an enemy of humanity, became an ally of the Master Chief and humanity after realizing the Covenant was based on lies and deception. Sergeant Johnson served throughout the Human-Covenant War, being the only person to fight in the first and last battle of the war. Franklin Mendez trained both the Spartan-II and Spartan-III programs, and was a highly decorated soldier. John Forge’s military career was short, sacrificing himself to save the entire crew of the UNSC Spirit of Fire. Veta Lopis’ military career started late into her life, but she is a strong leader of the Spartan-IIIs in Ferret Team Gamma.
Doctor Catherine Halsey worked on many secret projects throughout her time with the Office of Naval Intelligence, but her most infamous one was the Spartan-II program, which would not have been possible without her involvement. Professor Ellen Anders was considered as brilliant as Doctor Halsey by many, and her involvement in the Battle of the Etran Harborage and the Second Ark Conflict was the difference between defeat and victory.
There are too many AIs in the UNSC to count, but these AIs are the most influential and likely smartest of them all. Cortana is well known for her abilities while working with John-117, and also her betrayal of humanity in 2558. Roland is loyal to the core, serving as the UNSC Infinity’s shipboard AI. Isabel was never designed to be a military AI, but has fit into the role easily due to her powerful intellect. Serina was possibly the smartest AI of her time, with much of her talents going to waste after the Spirit of Fire was left adrift. Black-Box may be the smartest AI of the post-war era, though his intelligence may well be rivaled by Cortana.
The Librarian always liked to work from the shadows, and did so even after death, helping the Master Chief defeat the Ur-Didact 100,000 years after she died. The IsoDidact did not choose his role, but he accepted it and became the leader the galaxy needed in the last days of the Forerunners.