Of course this topic/poll is subjective and just a matter of personal opinion. I voted for the year of 1975.
This is why: I believe that Led Zeppelin were at their absolute artistic peak and zenith during this year. For two reasons. The release of Physical Grafitti and their subsequent 1975 American tour (along with the five Earl's Court concerts in May). I realize that 1975 also brought much pain and sorrow to Robert Plant (and His Family) as well as the other members of Led Zeppelin. However, I think, that 1975 was the year that (finally) gave Led Zeppelin the Prestige and Merit that They actually deserved, in the eyes of all the critics and sceptics that ever doubted their Power and Mystery (and Hammer of the Gods).
Speaking of Power, Mystery, Merit and Prestige, I think that the year of 2012 renewed all of that for all the World to see. The release of the O2 concert on cd/dvd/blu-ray and the Ultimate Acclaim of Led Zeppelin being Honored in the United States of America at last months 35th annual Kennedy Center Honors Awards.
From the first notes of Good Times, Bad Times in 1968/1969, til this very moment, IT MAKES ME SO PROUD TO BE A FAN OF LED ZEPPELIN (JIMMY PAGE/ROBERT PLANT/JOHN PAUL JONES AND JOHN HENRY BONHAM).
LED ZEPPELIN AND THEIR MUSIC WILL BE FOREVER REMEMBERED A 1,000 YEARS FROM NOW.
LONG LIVE THE "MIGHTY LED ZEPPELIN"!!!
(YOU HAVE DEFINITELY CHANGED MY LIFE FOR THE BETTER).