My favorite mission is The Breaking. The fight/climb throughout the Domain to reach Cortana while her and the Warden Eternal are talking to you and the Wardens fighting you are nice, but the best part of that mission has got to be in the latter part where the Warden says "Humanities discovery of the Halo rings was the culmination of the Librarians planning. Humanity was ready to take the Mantle." and the Halo music theme playing in the background with the Blue Team music theme was just amazing!
Guardians was an awesome mission. It had great sandbox action, character banter (Is she calling me old?), universe altering events, and some sweet music in it. I also found it amusing that Cortana refers to Jameson Locke as Mister Locke.