1. "The Time Lord Victorious Is Wrong." "That's For Me To decide."
It's a scene legendary in NewWho: David Tennant in his orange spacesuit, standing over the fiery control pit of a crumbling base on Mars; Lindsay Duncan deploring him to stop, a look of manic, Master-like excitement in his eye. 'The Waters of Mars' though isn't a Marvel comic rip-off of the great scientist becoming tired of watching everything die around him. It's greyer than that. Good and evil simply aren't definable in this case, just like with the destruction of the Family; here, the Doctor's arrogance is leading him to do something morally grey. The Time Lord Victorious scene is one of the most famous in New Who history, because it shows the Doctor at the start of a new path. A path where he'd bend the laws of time to his will and forget the pain of committing double genocide. This is a Doctor we don't recognise, and for once, we're not on his side. What makes the Doctor scary in this scene though isn't what he's become. It's his willingness to go further. He declares himself the "winner" of his species. He shows a total disregard of altering a fixed point in time. If Adelaide hadn't have brought him back to earth with her death, who knows how he'd have reacted to the Time Lords returning? This is the Doctor's most villainous scene, because it's frighteningly believable. He's about to turn evil - if anything, it's a wonder he didn't turn to evil sooner - and it feels real.