I wasn't expecting it to be at the time but after completing all of the 60s I declared that 1968 was definitely the strongest year and the weakest was 1962.
1969 had a weak start but then picked up. 1971 also has an extremely slow start.
Difficult to judge 1970 as it has a few totally outstanding songs in it.
1970 was my least fave year for sheer numbers of great songs largely because I was out of the country for all of it and you'd have to go back to the early 60's to be on a low par. A few years back I reassessed it properly when I got hold of a US Top 40 charts book and dug out loads of great songs I'd never heard, and it's pretty good actually. Ironically, I know the US charts of 69 and 71 less well as I havent reviewed them yet, so really looking forward to discoveries!