Like everyone else, consideration must go to bands/albums who took me from Point A to Point B in terms of musical taste... and in this regard Fear Factory's "Obsolete" was very important for me. When I came to realize that the aspects of the style I particularly appreciated were being done, and had been done, better, and as part of a vast expanse of styles of bands primarily based in Scandanavia, I looked into Hypocrisy, Edge of Sanity and Opeth and came to embrace the underground fully. It bridged the gap for me from "mainstream" North American metal, to, well, the good stuff.
Frank Zappa, obviously, was very important for me to realize the distinctions between music as art and entertainment, and the endless possiblities of musical style. "We're Only in it for the Money" and "Hot Rats" stand as strong achievements in my proverbial personal musical dictionary.
Pink Floyd (primarily Dark Side and The Wall), Led Zeppelin (first 6 albums) and Black Sabbath (first 6 or so albums) were key milestones on my musical exploratory journey.
And, of course, sitting atop my "opinion apex", if you will, is Opeth, with My Arms Your Hearse and Morningrise probably 1 and 2 as my fave records ever.