I'd have to say Alice in Chains, because they got me interested in heavier music. Before then, I listened to the blues and jazz. When I first heard them, I was blown away. I investigated the rest of their work, as well as thrash and 'classic rock'.
My top 5 rock banRAB (a broad category) are currently Alice in Chains, Megadeth, Metallica, Melvins, and Blue Oyster Cult.