I have to go with IV, just out of the sheer amount of good shit on it. Black Dog, Rock and Roll, The Battle of Evermore, Stairway to Heaven, Misty Mountain Hop, Four Sticks, Going to California, When the Levee Breaks. Except for Four Sticks, I LOVE every song on that album!
Let record sales speak for themselves Led Zeppelin IV: 23x Platinum Physical Graphitti: 16x Platinum Led Zeppelin II: 12x Platinum House of the Holy: 11x Platinum Led Zeppelin I: 8x Platinum
Physical Graffiti. They are reuniting for a concert on December 10th. Maybe we might have more to look forward too. One can only hope.