Fenris beat me to it – Fantastic Four #50, with the climax of the “Galactus Trilogy”. Heck, there are a lot of issues of the “classic” FF (written by Stan Lee/ illustrated by Jack Kirby) that I’d include here, but this is probably the best choice.
Thor 380 where he finally fulfils the ancient prophecy and fights to the death against the Midgard serpent Jormugand.