airplane

Kent is correct...however...if you wanted to try the odds, sit on the right side of the plane.

Heathrow has two parallel runways that run east to west.

If arrivals from the east, London will be on your right.

If arrivals from the west, you'll get a great view of the English countryside and be nowhere near London. (Heathrow is west/sw of London)

The pilot won't know which Runway is in use until he has begun his approach into London

It cannot be reliably predicted at Heathrow, because it depends on at least 3 things that none of us can know in advance. I was a pilot (not an airline pilot) and am familiar with the layout of the Heathrow runways in relation to London. It depends on the direction from which the wind is blowing as your airplane makes its approach, the weather at the time (instrument or visual approach), and the particular approach path assigned by air traffic control. There are airports, such as LAX, where the wind is almost always from one direction and the approach paths are 80% predictable, but that is not the case at Heathrow. Flip a coin.Your best aerial view of London will be from the top of the London Eye, go there for your view!