1978 for me has two monster albums:
1 - BRUCE SPRINGSTEEN - Darkness on the Edge of Town
One of his best acclaimed albums, possibly my favorite overall along with Born to Run (especially thinking of the live versions), the incredibly consistent The Rising and also Magic. A dark pearl, more somber than the previous album just like Awake is darker than Images and Words, and full of legendary songs.
2 - RAINBOW - Long Live Rock n' Roll
It's always hard to decide which one is the best Rainbow album because each of the three Dio albums has absolutely legendary tracks, milestone of the genres to be preserved once the Sun will eat up our planet, and some duds here and there. Needless to say that Long Live Rock n' Roll, Kill the King, Rainbow Eyes and Gates of Babylon are all amazing tracks and this album is a great gem.
Rounding it out we have:
3 - ALICE COOPER - From the Inside
One of his best albums of his early carrer, and one of the most forgotten ones. So many great songs, collection of tales from the mental ward.
4 - JUDAS PRIEST - Stained Class
Not a big fan overall, but speaking of legendary tracks, Beyond the Realms of Death should make several "best song ever" lists.
5 - QUEEN - Jazz
Wikipedia also says that "Killing Machine" by Judas Priest was released in 1978, then the album with the new title Hell Bent for Leather was released as well in 1979. Dunno how to count it.