I think the best/most monumental years were probably 1967, 1969, 1970, 1971, 1977, 1979, 1991 and 1994. All those years seemed to be eventful in spawning massively acclaimed albums and influencial genres, at least in the rock genre (1967 - psychedelic rock, 1969 - was just a really good year, 1970 - ditto, 1971 - glam rock and progressive rock, 1977 - punk, 1979 - post-punk, 1989 - Madchester/shoegazing, 1991 - grunge, 1994 - britpop). Don't know a lot about the history of other genres, you'll have to fill me in on them.
I'd say the worst/least eventful years were 1974, 1976, the majority of the 80s (especially the early and mid 80s), 1996, 1998, 2000, 2005 and 2006. Especially 1974, can't honestly think of a single iconic album that came out in that year.
What d'you think?