Hmm tough question...
Pantera- Great Southern Trendkill (This is one of the albums that got me into metal)
Slipknot- Iowa To me this album really showed that "new school" metal could exist and be awesome.
In Flames- Reroute to Remain Always just been my favorite In Flames album and they are one of my top 5 fav. bands
Gojira- L'enfant Sauvage (This album is amazing)
Metallica- Ride the Lightning/And Justice for all these are 1a and 1b for me with regards to their albums.
Children of Bodom- Follow the Reaper literally melts my face.
Opeth- Blackwater Park I love Opeth this to me is where they hit their peak.
Arch Enemy- Anthems of Rebellion My first introduction to a female metal singer...so amazing.
Five Finger Death Punch- The Way of the Fist (I know a lot of "true metal fans" hate FFDP but I have been a fan of them forever. The first show of theirs I saw cost me $7 and was one of the best shows I had ever seen) Love the pits for these guys.
AC/DC- Back in Black (I will be honest I have always thought of AC/DC as hard rock given my age but for the era that this album was released it was considered metal...and I absolutely love this album so I am going with the "metal for its time attitude)