Depends on how I'm feeling, and also on what "favourite line" means - some lines stick in your head, and some lines feel good to sing or to say, and they aren't necessarily the same lines.

And sometimes when you check on it the lines that stick in your head it turns out you've changed them.

One like that I thought I couldn't get out of my head was a Johnny Cash song "Beer for breakfast once again" which turned out to be "Beans for breakfast once again, it's hard to eat them from the can".

And there's words that sum up a whole story succinctly - Hank Williams "My son calls another man Daddy".

And another is a song I was haunted by for years just because of the line "I still remember what was on the jukebox when she turned", which was from a song Nic Jones sang. And seing this thread set me thinking about that, and if you open a thread that was at time of writing just next to this, called "Obit: Jonathan Eberhart", and scroll to the end, you'll see a story about where that led me.