or Rai

Maybe Ami but since she and Mina are like twins, similar in personality, I think they could be pretty close.

Nvm. I am just watching the live-action series Pretty Guardian Sailor Moon and Ami definitely seems closer to Usagi to me. She seems to worry more for her and she doesn't seem to oppose her or make fun of her. She's just always smiling at her.

i agree with tigerranma that ami is definitely the closest to usagi other than mamoru. Ami is always comforting usagi and seems to care for her more than rei and the others do. Plus usagi seems to care about ami the most even more than rei.

there was a couple of episodes where usagi really did care for ami. One is the one in the r series where ami thinks about moving away elsewhere to study her dream in becoming a doctor (and usagi was really worried about her). Another is the s series where ami was feeling really down and usagi tries to think of ways to cheer ami up.

and i agree that ami is way closer to serena in the live action series than the anime and manga combined!

rie is definetily the cloosest

Definitely.

Ami-chan seems the closest to Usagi..then Rei-san even with their fighting sometimes that still can bring them closer together and that's probably the way they show their friendship by fighting then Mako-chan..