Piccadilly line
The Piccadilly line is a deep level underground line, which stretches from the north to the west of London, serving 53 stations. It has two branches splitting the Westbound trains at the station of Acton Town and further going to Uxbridge and Heathrow Airport. The colour of the line on the tube map is dark blue, officially - Corporate blue.
The line was opened in December 1906. At the time of opening it ran from Finsbury Park to Hammersmith, it was extended to Cockfosters in the north, and Uxbridge and Hounslow West in the west in the early 1930s. Further extensions to serve Heathrow Airport happened in 1977 to Heathrow Terminals 1, 2, 3 (Heathrow Central at the time) and in 1986 to Heathrow Terminal 4 with a one-way loop to Heathrow Terminals 1, 2, 3. The latest extension on the line was to Heathrow Terminal 5 in 2008.
This line has a special place in my heart ♥️, as it was the first line I ever rode on the London metro back in 2011 when I came to London for the first time. It is also the line I have used the most among all the lines - I traveled from Hounslow Central to Holborn every weekday to attend my English classes for 1.5 months. Even the woman's voice announcing the stations feels dear to my ear 🤩, not like at other lines, which sound a bit strange to me.
Below are the descriptions and pictures of the stations on the Piccadilly line.

List of the Piccadilly line stations
Hatton Cross
Hounslow West
Hounslow Central
Hounslow East
Osterley
Boston Manor
Northfields
South Ealing
Acton Town