Very difficult choice this. But one thing springs to mind.
The sequence in 'Rose' where the Doctor and Rose are looking for the Nestene transmitter. The Doctor describes what it must look like, with the London Eye behind him.
Rose nods her head towards it. He turns around, looks for a moment, then turns back, having missed it entirely.
She gestures again.
He looks again, and finally sees it.
They dash across the bridge, hand in hand, with that wonderful up tempo tune.
Not the greatest scene in Who history. But that is what convinced me, back in 2005, that this new iteration was still the same daft, brilliant, glorious old nonsense I'd loved as a child.