Which Led Zeppelin I-IV album best defines their hard rock founding era?

Hard Rock Founding Era (1968-1971) poll options

Hard Rock Founding Era (1968-1971)

Led Zeppelin: Led Zeppelin to Led Zeppelin IV Era (1968-1971)

Led Zeppelin

• Led Zeppelin I set the blueprint
• II perfected the heavy attack
• III showed their boldest growth
• IV became the ultimate statement