I followed a similar trend around third grade or so. Got up to something in the mid-thirties. My favorite was the Thanksgiving one. Classic.
Same here. "The Spider Sapphire Mystery" was my favourite, but for some reason I don't remember the story at all. I think I was obsessed more with the cover at that time which is why I read it so much. I first got into Nancy Drew in grade 3 when my library gave away free books (renovating) and I got a couple.
Nancy Drew was the first novel series I got into, but before that I was absolutely obsessed with Archie comics.