Rocker440 are we talking debut albums or the album that really catapulted the band ??
I think Demons And Wizards really put Uriah Heep onto another level even though it's their 4th album.
However I really rate the debut as well.
Either or possibly both, Sox. Any album that you believe to be the album that established a band as solid, or successful that also brought them success and/or recognition as a band or artist, in your opinion. Some breakthrough albums may be debuts or from another point throughout a bands career. Demons And Wizards is spot on, and the album that got my attention. From the first song through to the last, I like that album! Although I heard Look At Yourself before Demons And Wizards and I'd probably choose LaY as my choice for UH's breakthrough album.
There certainly are many great bands, albums and points for all of the albums posted here, and I both agree, and enjoy many of these same albums, too!
A couple more excellent, classic albums that were both breakthrough albums and albums that I was extremely impressed with when I first heard them, and remain favorites.

Spirit - Twelve Dreams Of Dr. Sardonicus
Although I had heard a couple of other Spirit songs before Twelve Dreams, it was this Spirit album that caught my attention and I consider to be their breakthrough album! To this day I enjoy listening to this fantastic album, and thinking of many different things, places, times and people!

BTO - Not Fragile
I see that LG posted BTO - II!

My choice, Not Fragile, is probably the same as many other folks. That album was both a breakthrough album, but also a tremendous success for BTO, imo! When Not Fragile was first released most of the people I knew at that time talked a lot about the album, with mostly praises about the great songs, music and how impressive the entire album was to many of them, and I certainly agreed then and do right now, too!

Blue Oyster Cult - S/T
The excellent debut album of Blue Oyster Cult hooked me when I first hear it many years ago, and in various good ways. Many of the songs impressed me then and are long time favorites now. I thoroughly enjoy this album and listen to it as often as possible, with much emphasis on volume, too!
hawk, Van Halen's debut is a fantastic breakthrough album, imo! The comments I made with regard to Not Fragile also applied to Van Halen's debut!
