You would argue this over London Calling?
Can't BOTH be essential?
I'd consider an "essential" album to be something of a landmark. London Calling is certainly a landmark album; nothing else the Clash did--regardless of quality--is a milestone.
Sometimes it doesn't have to be your favorite album by a particular band. What's generally considered the essential VU album is the first one, and, while I like it a great deal, I prefer White Light/White Heat and the self-titled third record. Still, I wouldn't deny The Velvet Underground & Nico its place in the sun.
Oh, I haven't noticed it in this thread (and if it has been mentioned, I apologize), but Miles Davis' Kind of Blue is The Jazz Album to Start With.