The Metropolitan Line was the first line but has sections of overground running.
The first deep level tube that still runs in (more or less!) its original format is a section of line that is now part of the Northern Line.
Over the years the track sections have changed hands a number of times and different sections have operated under one or more line names so it is not always easy to define the 'first' of any of the Underground's achievements.
The Central Line
The Northern Line.
The Circle Line.
District Line (London Underground) from Hammersmith tube station to Victoria tube station.
Camden
Camden
The Central line is the current oldest tube stock in the whole of the London Underground.
The Circle Line Unfortunately this answer is wrong! According to Transport for London the only tube line to connect all London Underground lines is the Jubilee Line.
Depends on the context: London has a tube (Metro) line which is called the Circle Line.
The section of tube between Edgeware and Farringdon which is now part of both the Circle and Hammersmith & City lines
Redbridge on the Central Line - which is also coloured red on the tube map.
Presumably you mean the London Tube? If so, it is the Jubilee Line (silver).