Man, tough question. I guess I would say 1987. That year had about 55 NES games released. I call 9 of them great:
legend of zelda; castlevania; punch-out!; kid niki; bomberman; gradius; mega man; ikari warriors; kid icarus
1989 only had 8 great games in my book, out of 97:
mega man 2; ducktales; monster party; shadowgate; tetris; adventures of lolo; adventures of bayou billy; marble madness
1990 also had 9 great games. But that's out of 177 games released--way more than any other year.
super mario 3; castlevania 3; double dragon 2; mega man 3; river city ransom; jackie chan's action kung fu; journey to silius; rescue rangers; snake rattle n roll
The ratio is so skewed. 9 out of 55 total is much better than 9 out of 177.
1988 was also a strong contender, but with only 6 great games out of 65, I had to pass it up:
contra; double dragon; super mario 2; bubble bobble; bionic commando; zelda 2
I got my numbers and years from
here. You can sort the list by release date.