What I did like was the Einstein mission from RA2 and YR. Because in this mission you pretty much swapped roles between the games. First you played the Allied to defend Einstein from the Soviet, but in YR you swapped your role to Soviet, where you had to destroy Einsteins lab instead.
It was also pretty fun to play against your ''Allied counterpart''. The AI pretty much played like a real player. Lots of expanding, sending good amount of troops, great defence. They even build around Einsteins lab, thats what a real player should do and not an average AI.