The Abominable Bride - which starred Benedict Cumberbatch and Martin Freeman as Sherlock Holmes and Dr Watson - attracted 34.7% of the TV audience.
The final episode of ITV drama Downton Abbey won the Christmas Day ratings battle with an average of 6.6m viewers.
BBC One's New Year's Eve fireworks was watched by more than 12m, the BBC said.
Sherlock attracted 8.4m viewers, the highest overnight audience across all channels for a drama of the festive season, the BBC said.
The two-hour Downton Abbey finale attracted a peak audience of 7.1m, beating the Strictly Come Dancing Christmas special as the most watched Christmas Day show.
The Strictly special was the BBC's most-watched show of Christmas Day, with a peak of seven million viewers and an average of 6.5m.

Twenty tonnes of pasta covered the A5 near Shotatton, forcing police to close the road between the A483 at Oswestry and the A458 at Shrewsbury.
A spokeswoman for West Mercia Police, which attended just after 11:30 BST, said it was not clear how the pasta had ended up on the road.
No-one was arrested and no-one was injured.
A Highways England spokesman said: "We sent people there with high pressure hoses to clean it up but it does seem that it got everywhere.
"All of it has needed sucking up and taking away.
"It's been quite a clean-up job."
BBC One show Sherlock attracted more than eight million viewers for its New Year's Day special - the highest TV drama audience of the festive season.
[NEXT_CONCEPT]
A major road in Shropshire was closed after a lorry shed its load of pre-made spaghetti bolognese.