Bands I grew up with so I refuse to get into the whole 'best band of all time' thing since opinions are just that, subjective:
Pantera (and later Down, but it's more Southern/sludge metal, Damageplan didn't do it for me though and RIP Dimebag and Vinne Paul)
Sepultura pre-Igor Cavalera leaving
pre-Black Album Metallica
Megadeth
Motorhead
Iron Maiden
Ministry (industrial metal)
Anthrax (and S.O.D.)
Helmet
Prong
Biohazard
White Zombie
Slayer
Ozzy
Crowbar (also sludge metal)
Corrosion of Conformity
Danzig to a degree is metal IMO
GWAR (I was stained green for a few days after seeing 'em live lol)
Testament (some of the guys at my school also were into Manowar, but I found them to be goofy as hell)
Exodus
King's X
My skate rat buddy had me check out D.R.I. and Suicidal Tendencies (who I really liked)
Ice T gave a solid effort w/ Body Count
Probably the heaviest show I've been to was Motorhead w/ Morbid Angel at UNO on the floor in high school lol.
Outside of Morbid Angel and Obituary, I really didn't stray much into death metal.
Rage Against the Machine def had some metal elements in some of their songs, they def pioneered 'rap metal' which probably is part of what led to some of nu metal i.e. KoRn (their first album is one of my all time favorite albums by any band)
Early Deftones were heavy as hell, and they remain one of my favorite bands to this day.
Tool can be sonically crushing so they have metal in their DNA, I call them 'math metal' b/c of how complex their signatures are and how songs switch between them so flawlessly. Hell, the song Lateralus uses the Fibonacci Sequence and the Golden Ratio.
More recent (as in after that first main list) that I listen to:
Slipknot/Stone Sour
Mudvayne
Sevendust
Meshuggah
Jinjer
Mastodon
Lamb of God
Hatebreed
Gojira
The Dillinger Escape Plan
As I Lay Dying
Parkway Drive
System of a Down