One issue that will always hold a place in my heart would have to be Amazing Spiderman #351. It's the first issue that Mark Bagley came on as full
time artist for the series after Erik Larsen went and helped form Image. It also has a really good storyline with the Tri Sentinal and an apperance
by Nova, but moreover it was the first issue I payed for with my own money!
Dunno if this counts but Watchmen is still my all time favourite.
There have been some that have come pretty close but watchmen still rules all.
Can't wait for the movie!