Ye Olde Cheshire Cheese, Fleet Street
It has some great history to it and considering its tucked down a little side alley, you would never think its as big as it is! It's massive! Multiple different rooms and bars, a huge cellar bar, restaurants, saloon bar etc.
The staff all seemed nice and being a Samuel Smith pub the drinks are very well priced too!
A mate of mine recommended a new iPhone pub-finder app called "Smith Finder" http://bit.ly/TxB5ki that features this pub in it and loads more like it. Great for doing a bit of a pub crawl around London.
145 Fleet Street, Fleet Street, London, EC4A 2BU
From Blackfriars: leave the tube and head up Blackfriars road (away from the river) towards Ludgate circus. Turn left into Fleet Street and the pub is a about 3 mins walk up Fleet Street, on your right. From Chancery lane: leave the tube and head east towards Holborn Circus. Turn right down Fetter lane before Holborn Circus and walk to the end. Continue down New Fetter lane to Fleet Street. Turn left into Fleet Street and the pub will be on your left a short distance in front of you. The entrance is the left hand passage way as you face the pub. If you use the other one you end up in Scruffy Murphy's.