8. River Tam
The truth is, if the show had been able to continue, River would likely have ranked much higher. She is an unbelievably important part of the show's puzzle, as the mystery surrounding her experimentation and her evident prowess as a weapon was a central narrative in the show. However, the inaugural season doesn't fully delve into it, as they clearly planned to get to it later, and it leaves River feeling a little like a joke without the punchline.
Summer Glau's presence is definitely felt, as River becomes an equal source of comedy and dread. Her sporadic mood shifts and dangerous encounters make you feel for her innocence, whilst still being afraid of what she could do to the crew. This made her an always fun element of the show, as she brought a domestic danger to the characters.
However, her unfinished story isn't the characters only issue, as River is often too disjointed to understand. Her manic behaviour ends up limiting her development, as audiences are only given momentary glimpses into who she is/was before her tragic experiences with the Alliance Parliament, leaving her too much of a mystery to bond with fully.