I still gotta go with #300. Impaling Arishem with the Odinsword and basically lasting as long as an Odin-powered Destroyer was great. Many great issues since then, but that moment/feat was great.
Thor 300 was great. I read it for the first time a few years ago and really liked it.
When I was a kid, Thor Vs Dracula in Thor 332-333 was an event to me. I'm not sure how it holds up today, but I still have fond memories of it.
I really thought the first appearance of the Absorbing Man was great, in Thor 114-115. He was actually scary.