Ok I think the choice is clear for me. The album that had the most impact on me
in 1991 (as opposed to some others I may not have heard till a year or two later) was:
1. Blood Sugar Sex Magik by Red Hot Chili Peppers
I used to draw Anthony's asterisk tattoo on my wrist with a black pen day after day whenever it started to wear out, for Christ's sake. I remember spending a lot of time admiring the booklet photos, reading lyrics, etc. Now sure the album hasn't aged all that well, some songs have unfortunately been overplayed to death ('Under the Bridge' used to sound completely different to me before it was released as a single, and within a week or two it had turned into a different song somehow). But for me this isn't about which album I would pick up first today, which I could play nonstop, it's about which meant the most to me
then and still holds up as a great album.
Honorable mentions:
Smashing Pumpkins - Gish (didn't hear till 92)
Nirvana - Nevermind
Primus - Sailing the Seas of Cheese
Mr. Bungle - Mr. Bungle (didn't hear till 92/93)
Fugazi - Steady Diet of Nothing (didn't hear till 93)