Mel Bush

Melanie Jane Bush, BSc (Hons) is possibly the most powerful character ever to appear onscreen. Her timeline makes literally no kind of sense but that's only because if Doccy Who attempted to explain it everyone's brains would explode due to the nature of Mel being beyond human comprehension.
Mel is notable for having fantastic taste in music, canonically being a fan of ABBA. She is also notable for having all the braincells, and for being the most valid person alive.