I really liked the original, Susan. I've only seen a few of her episodes (the ones on Netflix), and I really like her. She serves as a great contrast to Hartnell's Doctor. I also love Jack, because he's, well, Jack. Donna would also have to be a favorite, because she's smart, funny, and doesn't take any of the Doctor's crap.
It so hard to just pick one...but I think my favorite companion is Donna Noble. She's sassy, spunky, compassionate and completely brilliant. She's the most important woman in creation. So cool reality bends around her. And I love her sisterly relationship with the Doctor.
I am also a huge Sarah Jane Smith fan. I just love her relationship with the Doctor. "Have you met Miss Smith? She's my best friend." Plus she was with four regenerations of the Doctor!
Jack counts! He's a companion. He's not a gal, but I'm sure under the right circumstances he can scream like a girl.
definitely river song, or Donna. its too hard to choose!
I am utterly in love with Amy.
I am completely for Rose Tyler. I'm not the only one that cried watching Doomsday, am I?