Piccadilly Line

if you are going to Heathrow Airport then train to use is Piccadilly Line on Underground

runs from Cockfosters zone 5 from North London and through to central london ends at Heathrow Terminals 1,2,3 and 4.

one of the bussiest networks as well. and covers following major areas

Wood Green
Manor HOuse
Arsenal
Holloway Road
Finsbury Park
Kings Cross st Pancras
Holborn
Lecister Square
Piccadilly Circus
Hyde Park
Knightsbridge
Gloucester Road
Earl's Court
Acton Town
Honslow
and
Heathrow 1 2 3 and 4 terminals