I don't know the recent candidate (probably a pepsi ad given the amount of over-rated expensive football players in them) but when it was made I'm pretty sure the Volksawagen advert for the new polo (1990's model) when they had it falling of a tower block, was the most expensive.
I have a feeling it's the British Airways one from the mid 80's.
Where all the people form a flag at the end.
Whatched a documentry on it once, and I know it cost rather alot.
The Guinness advert with the surfers waiting for the wave which is shown as galloping horses cost over a million pounds and was reported in several papers as being the most expensive advert on British television.