Metallica (2003) - The first CD I ever had (that got me into music as a whole) was a mix CD that I made from my brother's music collection that had four Metallica songs on it (Ride the Lightning, For Whom the Bell Tolls, Master of Puppets and Orion). Metallica has been one of my favorite bands since.
Linkin Park (2003) - That aforementioned mix CD had two Linkin Park songs (In the End and Runaway) on it as well and Linkin Park was probably my favorite band in middle school and the first part of high school. I stopped liking them once that shitty Jay-Z collaboration came out, but rediscovered them probably around my Senior year of high school.
Slayer (2004) - Hearing Seasons in the Abyss pretty much cemented metal as my favorite genre of all time. Life changing.
Rob Zombie (2005) - Hellbilly Deluxe was the first album I ever bought with my own money and still remains one of my favorite albums of all time to this day.
Cannibal Corpse, Death and Obituary (2006) I heard all these bands at about the same time and they really got me into death metal. I randomly downloaded "Fucked with a Knife" from Limewire because the title made me laugh. I saw some senior at school wearing a Scream Bloody Gore shirt and I had to know what that sounded like, so I went home and downloaded the title track. I was listening to the metal station on Music Choice or something like that and the song "Insane" by Obituary came on and I was instantly hooked on the band. Cause of Death was my shit in high school.
Anti-Flag and Bad Religion (2006) Thanks to the albums Underground Network and No Control, I went on a punk binge for about a year in my Sophomore/Junior years of high school and still have an avid love for the genre.
The Prodigy (2009) - The local alternative station (RIP Q101
) used to play "Smack My Bitch Up" and "Firestarter" a lot and it was a breath of fresh air from some of the stuff they played. The Prodigy really opened me up to electronic music as a whole.
Dark Angel (2012) - I listened to Dark Angel very sparingly prior to buying Darkness Descends last year, but Darkness Descends soon became one of my favorite albums of all time. This also pretty much (with the help of Morbid Saint) made thrash overtake death metal as my favorite subgenre.