Just after regenerating, the Eleventh Doctor has a few lines.
DOCTOR: There's something else. Something important. I'm...I'm...I'm...crashing! Ha ha! Woo hoo hoo! Geronimo!
This is from the episode The End of Time: Part Two. This episode was written by Russell T. Davies and indeed was his last episode on the show as lead writer and executive producer.
'Geronimo' went on to become something of a catchphrase for the Eleventh Doctor. He uses it in plenty of other episodes.
So was that term and its place in the episode devised by Davies? Written by Steven Moffat? Improvised by Matt Smith? It perhaps wouldnt be surprising if Moffat wrote the final part of the scene since it was 'his' Doctor and he was taking over as lead writer for series 5. However, Davies is the only credited writer for the episode.
Who decided to use 'Geronimo' in this scene - Davies, Moffat or Smith?