Usagi

Ok, so maybe Usagi does end up being ChibiUsa's mother in the 30th century, but since they are a constant part and annoyance in each other's lives in present Tokyo, Usagi should be included as one of ChibiUsa's dearest friends.
  In SailorMoon R, ChibiUsa hypnotizes Usagi's family into thinking she's Usagi's cousin, and ends up living in the Tsukino household for quite a while. ChibiUsa and Usagi become famous for squabbling with each other ... maybe even more than with Rei. Yet, they share their special moments ... after all, ChibiUsa is her daughter!
  The unpleasant relationship takes foot when they first meet, of course. At the time, ChibiUsa is very young and you have to admit that she has the tendency to be agonizing. The actual depth of their relationship is apparent though, as SailorMoon continuously risks her life for ChibiUsa's sake. As Usagi describes ... "a part of me wants to protect her, and a part of me wants to get rid of her." Usagi is already partially knowledgeable of her future motherhood. At the end of the season, Usagi, embarrassed, hugs ChibiUsa good-bye as she calls her "mommy."
  In SailorStars, SailorMoon even jumps of a building to save a falling ChibiUsa, and is the first to bawl when she announces she's returning to Crystal Tokyo.
  Usagi is very affectionate towards ChibiUsa (really!), and the relationship grows stronger as the series progresses. Even though ChibiUsa constantly hogs Mamo-chan away ... regardless of their rivalry, SailorMoon and ChibiMoon share the strongest and most important bond in all of Bishoujo Senshi SailorMoon.