I'm not a massive Clash fan and only know the big hits so would go with Rock the Casbah. As for The Jam - Down in the Tube Station at Midnight. Now, I know this isn't the original, but I do think it's one of the best cover versions I've ever heard. It really does the original justice, but is indelibly a Carter USM track when performed by them.
I went to see Carter once, as I was seeing an 18 year old nurse, after my marriage ended & she was a big fan. Apart from Sheriff Fatman, could take or leave them.
Guns of Brixton - Paul Simonon doing a cracking job - The Butterfly Collector - not a bad cover version either -
I think all my choices for the Jam have been put on here already. Clash I find to be overrated but then I saw some footage of some of their early gigs and was astounded, the energy was amazing, bordering on raw violence. Not something you could keep up of course and musically limited but they had real punk credentials, not the pub rock band I thought they were.
Not bothered about Clash, but have every Jam album. Very difficult to have a favourite song, but Liza Radley is right up there for me.