re: Album of a band you would tell someone to buy first if they never heard them before

Yes - The Yes Album
Led Zeppelin - IV
Queen - A Night at the Opera
Rush - Moving Pictures
Van Halen - Van Halen
Black Sabbath - Heaven and Hell
UFO - Strangers in the Night
Peter Frampton - Frampton Comes Alive
Cheap Trick - at Budokan
AC/DC - If You Want Blood (You've Got It)
U2 - The Joshua Tree
Pearl Jam - Ten
Rage Against the Machine - Rage Against the Machine

Mostly really boring picks but I wanted to say that for some bands I think a live album is the best introduction.

Also I assume Heaven and Hell would be controversial since people associate Ozzy more with Black Sabbath, but Heaven and Hell was my introduction to the group so that's how I see them.