For me it is the years when I was too young to understand how things really worked at Disneyland, and in the real world I suppose. Being a kid who grew up here in So. Cal that took annual trips (during the late 70's and 80's) the day after Christmas really set the stage for a truly magical experience. I didn't really go that much during the 90's, maybe every other year or so? When I was younger I don't remember being bothered by lines, or bothered by the heat, all I cared about was the rides, Disneyland could do no wrong.
Now that I'm older (38), and am on my 4th year in a row of having an Annual Pass, I still love the park just as much as ever. Obviously I have some idea of how the DL and the real world works, but when I cross under the DLRR into Town Square, all of those thoughts and concerns of the real world disappear. I can't really pinpoint an exact best year, so I will stick with what I said in my opening sentence.