Top 5? Interesting challange... The order is a bit arbitrary:
1. Overhead IBC4 aurora (corona), May 1970, Saskatchewan
2. Comet Ikeya-Seki in the Canberra pre-dawn sky in October 1965
3. Total eclipse of the Sun, Gippsland, 1976
4. Comet McNaught
5. The night sky, early morning, January 1988, seen from Perisher
And there are others competing, like the Venus transit, my first view of the rings of Saturn, DY Crucis through a 12" GSO, my first view of NGC3532 through a small telescope, Omega Cen through Peter Bobroff's 20"...
To paraphrase an old SBS slogan: "The Universe is an amazing place"
DN