When it was A stock, the Met was my least liked line. However, that title got swiftly moved onto the Piccadilly line some time ago. Most of the reasons are already mentioned, so I won't go over them again. The service level on the line, especially in the evenings and on Sunday mornings, is absolutely awful even when things are running normally, and the overcrowding that causes is insane. It is one line that needs a dire amount of investment and sharpish. The Bakerloo does too, but I can tolerate that line and it's ancient, clapped out trains. A ride on the Piccadilly line is less enjoyable than going to work...
I went on the Picc the other week, due to a farce on the WCML, and it was fine to begin with, got to the S stock depot (Acton depot? Never checked its name!) a little before Acton Town and to a halt we eventually came. Edged up, stopped. Edged up again, stopped, and these distances were beyond tiny. Eventually we got permission to move forward then slammed to a stop a few seconds later. After what was about 5 minutes after the first stop we FINALLY got into Acton Town and I was very happy to bail off there. I'd only joined the train at Alperton too, so it wasn't like I'd been on it for long!