If you were to only listen to one metal artist, it would be Strapping Young Lad/Devin Townsend Band (i said artist, not band).
Almost essential for the well rounded metalhead are:
Lamb of God
Byzantine
Gojira, they're "extreme metal"- but although all the elements are heavy the overall sound isn't that heavy to me. Very good songwriters.
And, yes it's a cliché- but revisit that Slipknot self titled album. (I persuaded myself that i didn't like slipknot or marilyn manson because they were what the 9 year old kids in hoodies liked. Since then i've got a grip and actually listened to their albums. And unfortunately for my street cred, they're damned good.)