KitKat and LittleAmy/littlebit are the two I miss most. Had I not rediscovered this site, I could've been on this list too. (Glad I did!)
I'm not going to list the two RU moderators who have standing accounts on here, although [Bubbles] hasn't been seen for a small while. (I do know she's okay though.) Plus, it's not uncommon for moderators on each site to visit the other (although I think I'm the only one, outside of Lois, to do it with regularity.)