My favorite song of their is "Straight to Hell", but it depends what you're into. Their eponymous first album is really good--it's pure punk with some reggae influence. Give 'em Enough Rope is also pretty good, bit more catchy. London Calling is a must--"Train in Vain," "Clampdown," and "Spanish Bombs" are all really good and easy to get into. Sandanista! is probably my favorite album of theirs--it's the most experimental and psychedelic. Combat Rock was the most commercial, but "Should I Stay or Should I Go" is great and "Rock the Casbah" is amazing. Anything after Topper was kicked out isn't really worth listening to (Cut the Crap, etc.)
My favorite Clash song from "London Calling," which I think is often overlooked (because it's such a damn good album) is Lost in the Supermarket I wasn't born, so much as I fell out......