I think the entire Soviet campaign in YR is the most memorable part of RA2.
In RA1... the 2nd and 4th Allied missions, and also the one where you had to rescue Tanya, especially because back then when I was a kid, I found that mission quite difficult. The first interior mission of the Soviets is something I remember often too. Actually, I wish that MO had interior missions like that.
The first two GDI missions in C&C1 are definitely quite memorable as well. Also the Greece mission, because to me, it was always the first real difficult one. Back then when I was a kid, the NOD disc went missing, so I only played the GDI missions over and over, and as such, they are the most memorable to me. Recently, I played the NOD missions the first time, and I really dislike most of them, because they are focused on commando missions. I do like the first NOD mission in the expansion, played that a million times. It's almost like playing skirmish.
Most of the time, the most memorable missions to me are the ones I got stuck for the longest. Like the one mission in Tiberian Sun when you had to recapture that GDI base with 4 Hover MLRS. Man, I hated that one.
In RA3, I think the final Soviet mission in Uprising is something that's quite special, as I never seen such gimmick used in RTS games before.
Am I the only one who definitely wants interior missions for Mental Omega?
The question is kinda weird, considering you quoted someone saying that they'd like interior missions in MO. :V