It had to be River. (Beware, this is a love letter.)
Because knowledge is power.
River Song is the “triumph of intellect and romance over brute force and cynicism”, not because intellect and romance proved more efficient or because she enjoys outsmarting people; she knows. She has “been a soldier too long to believe there are Gods watching over us” (The Pandorica Opens). She chose to resort to intellect rather than physical strength, only turning to violence when she has no choice and needs to prove a point.
Because she chose her field of study out of curiosity and carried on, probably enamoured with her discipline. Basically knowledge is romance. Because she became a professor, continued to learn and travel after being imprisoned, despite being a known criminal.
She’s also a woman I would want to know personally, not because she is fun, or beautiful, or skilled, or someone I would want to be, but because she baffles me still now, when her story seems to be over and done. She’s fundamentally flawed, for all her control; blind to her own needs, persuaded she is a psychopath, hazardously daredevil when it comes to her own life, trigger-happy, harsh, grandiose...
Because she never takes herself seriously, but rarely pushes fun to the point where you wonder who is starting to feel the need to take the situation in hand.
Because she owns the stage, not by being the centre attraction but by serving her partners with the right lines. Also she enjoys a good entrance and a good battle wit.
Because she started as an enamoured wide-eyed girl and ended up as a long-suffering married woman.
Because she puts her family first and actually learns not to put her love for the Doctor before the Universe –cf. the Big Bang vs. TWoRS. Because she shows it is okay to be grown up and need your dad.
Because she doesn’t rely on the Doctor, or anyone, to see the Universe and have fun; he is not her life.
Because you can be alone and hold your own and enjoy it and tease Sontarans on their hen night and still have a grand long life filled with love.
Because she is a proof that every so often life goes horribly wrong and the people you love cannot be there for you and that you have only yourself to support you, and she does. That everything changes, always; you cannot know what you are going to get. She goes on. And at the end, she is pretty content and happy with her life.
And River Song is a demi-goddess who lost her claim to immortality and worked to retrieve it by eventually becoming writer, and character, and narrator of her own life. From hero to Homer. She is a ghost and an immutable object, a siren and a ship. She is the Minotaur and Ariadne at once, Eurydice and Izanami, Isis and Persephone. Yes, she’s that layered.
River is a proof that women in Who are not there only when the Doctor is lacking, be it emotional intelligence or common sense. She’s there because she knows her job and does it well: she opens vaults and tombs -even after her death-, and leads expeditions underground, analyses what she is presented with to come up with backed up information –River actually rarely is shown picking information from her hat: she relies on her scanner readings, uses books and paper documents as reference (cf. TotA, Closing Time). Name your sources! Yep, she’s a scholar all right.
Because she is so different from the image society and media and even most girls want for women.
Because she controls and chooses her image and this is actually huge; River doesn’t follow trends. She’s practically the only character in New Who under Moffat bothering with time-appropriate or practical clothing –under RTD, it really depends on what are your views on blue jeans-, and yes, she has style still –and wears green and the entire world should always wear green. She’s all woman, outwardly, because she chooses to be so, when she wants to be so.
Which leads to the fact that she doesn’t need to be sweet and smiling. She can be grating and camp – from ‘you’ve got a mouth that won’t shut’ to ‘him indoors is being so useless-, but good for her. Women should not be asked to be nicer or more accommodating or even smiling on account of being female.
Finally because knowledge is power and River probably knows every point I disclose above. And is proud of that. She knows she’s good and wants you to know it, and that’s pretty refreshing for a female character.
So it had to be River and I'm forever grateful to Moffat for creating her.