Soilent Green, Anal ****, Celtic Frost, Morbid Angel, Napalm Death, (old) Slayer, (old) Iron Maiden... and of course, the Godfathers of Metal, Black Sabbath.
Does anybody know 'Before The Dawn'? It's a death metal band from Finland... One of the greatest I've heard in a long time.
Powerful chords, very melodic, great singers...
There is also Rootwater from Poland. (mix of In Flames, System Of A Down and Sepultura with folk influences).
King Diamond
Mercyful Fate
Nevermore
Meshuggah
Testament
Pantera
Iron Maiden
DIO
Daylight Dies
Opeth
Megadeth
Warbringer
Ihsahn
Strapping Young Lad
Sodom
Overkill
Bloodbath
I've been savoring a lot of symphonic and folk metal at the moment:
Nightwish
Edenbridge
Eluveitie
Finntroll
Kivimetsän Druidi
Dimmu Borgir
In Silentio Noctis
Rhapsody of Fire
Lyriel
Meshuggah
Abigail Williams
Ram Zet (one of my faves, one of the few extreme metal bands doing something different)
I could go on for ages, metal is such a varying genre of music that it's easy to find many bands that are highly enjoyable.