I’m going to exclude any hunters here because I wouldn’t consider them “ordinary humans” (just sayin’). I do like Charlie but not as much as I favour Sarah Blake, Sam’s love interest in 1.19 “Provenance” who then appears again in 8.22 “Clip Show” because she was really brave and calm when she was dealing with ghosts and then again when her life was a stake. She just seemed so relaxed and brave; a very cool human.
However (and I’m rating on “humanness”, similar to Sarah) I would have to say my favourite female human would be Lisa, Dean’s love interest from seasons 3-6. She was frickin awesome as humans go. She made a life for herself as a single mother and, despite Dean running off to fight demons and monsters, she kept herself grounded and kept things as normal and ordinary as possible for her son, Ben. In doing this, she was Dean’s rock throughout that year without his brother when he was trying to live a normal life, and she gave him comfort and support with whatever he was going through. She was also pretty clever and cool-headed when in the face of danger. If only we were all like her.