To answer the question in the headline: duh, obviously Westminster. But new info from the London Datastore about boroughs' daytime populations let us have some fun rootling around in the stats so we thought we'd share.
Westminster has 1,017,622 people spending significant amounts of time in it during the day; around 200,000 are tourists, so the next time you end up walking in the road to get past a slow moving group on your lunch break, remember it's far more likely you're muttering 'bloody tourists' at a fellow Londoner. Perhaps it's the 221,000 Westminster residents who are more entitled to mutter 'bloody commuter' at the rest of us that just work there.
If you want peace and quiet, head to Barking and Dagenham which has a daytime population of 190,382. The other two boroughs with less than 200,000 people during the day are Sutton and Kingston upon Thames. Other busy boroughs are the City (598,141), Camden (527,016), Southwark (462,473) and Tower Hamlets (428,085). Kensington and Chelsea has a daytime population of only 295,461, 65,000 of whom are tourists (though it might feel like more around South Ken) - which makes it quieter than Barnet, Brent, Bromley, Croydon, Ealing, Enfield, Hounslow, Islington, Lambeth, Wandsworth and Hillingdon. Ah, Hillingdon. We were initially surprised by the 400,000 people it attracts during the day, but then we remembered: Heathrow.
Photo by Che-burashka from the Londonist Flickr pool