Dream Theater - Awake (following I&W)
Marillion - Clutching at Straws (following Misplaced Childhood)
U2 - Achtung Baby (following Joshua Tree)
Journey - Frontiers (following Escape)
Nice thread and some really good ones here. Quoting the ones I'm wholeheartedly seconding.
I&W is my #1, but it's pretty amazing how DT came up with an album that's quite different and almost as good (in my book). Marillion too - in this case, Clutching might well be my favorite.
I'm adding a couple myself:
Paradise Lost - Draconian Times (following Icon) (Ok, maybe some people wouldn't considered Icon their landmark...perhaps Gothic was the truly "innovative" one, but Icon did put them on the map big time. And Draconian Times was basically as good.)
Tori Amos - Under the Pink (following Little Earthquakes)
Tool - Lateralus (following Ænima)...although maybe fans consider Lateralus more of a landmark?
Faith No More - Angel Dust (following The Real Thing)