Obviously there’s Amazing Fantasy #15.
Paul Jenkins had some great one-off issues in his Spectacular run. I believe it’s issues 21/22 which had a double whammy of great one-offs. One was just a poker game between the heroes and Kingpin joins in. It’s a fun issue where there isn’t any action, it’s just some guys playing poker. The other is about Mindworm and how he dies. It’s a beautiful and heartbreaking issue. Even the cover is sad as hell.
The kid who collects Spider-man is also great, but I think everyone says that. It’s one of the most common issues, but there’s a reason for it.
To have and to hold is another great issue. So much so that it was referenced at the start of Spencer’s run (which actually forced me round to reading it).
Lee and Ditko had some great issues too.