Ok, so this is kinda vague to be honest, Metal is quite diverse and doesnt not appeal to all people, What kind of metal?
Well you said all the way to modern so i will do a sum um of the various genres.
For the roots, Like everyone said, Black Sabbath is where it pretty much starts. But also check out Iron Maiden, King Diamond, Mercyful Fate, and Judas Priest, Maiden and Priest defined the dual guitar attack and the dual guitar harmony, a common metal signature. More good early bands are Saxon and Angel Witch, Rainbow and Dio.
Now in the 80s things got a bit darker and more evil, and with this, A LOT more heavy. The early 80s saw the birth of Black Metal, Death Metal, Thrash Metal, and Grind. You probably already know the main 80s thrash acts like Megadeth, Metallica, Anthrax, and Slayer, and all these bands are worth checking out, they released some of the best albums of all time. But there are some other guys such as Exodus, Testament, Tourniquet, Annihilator and Nuclear Assault that released incredibly fantastic albums, There are many more unknown acts i wont go into right now. (VinceG I believe you have a rather good list of unknown thrash acts in your head
) Other good Thrash acts are EVILE, Warbringer, Sanctity, and Kreator.
Now for death metal, this is hard for most people get into, mostly due to the extreme vocal stylings of Screams and Growls. If you want to ease your self into it, best thing is to listen to bands that use this with clean vocals, such as GMC's favorite Opeth, Novembers Doom, and even Amorphis, Once you have eased yourself in. You can try the more brutal stuff like Arch Enemy, Cannibal Corpse, Death, Suffocation, or Carcass, But the violent imagery of some of these acts is often misinterpreted, so you will have to keep an open mind when going into this, Give it all time, and a good chance, and the wonders of this music shall reveal themselves to you.
Another Genre, and one of my favorites is Progressive. Mostly long songs, complex structures, and intricate instrument work is the name of the game here. The most notable Prog Metal band is probably Dream Theater, But they are certainly not it. Nevermore, Amorphis, Opeth, Novembers Doom, Symphony X, Jag Panzer, are all bands that use similar ideas in different ways to produce very different sounds. All these bands focus on heavy riffs, guitar intricacy, and usually stick to just clean vocals, although Opeth, Novembers Doom, and Amorphis all use growling to one extreme or the other.
Perhaps the most female laden genre, Symphonic metal uses more aspects of Metal as whole than any other genre. Combining female clean vocals, male clean vocals, and growling from both male and female voices, Detuned guitars, large string arrangements, complex song structures, intricate solos and riffs, this genre is one of the most powerful.
Nightwish is easily the most well known Symphonic metal band, along with Epica, Therion, Within Temptation, Eyes Of Eden, Tristania, and The Sins Of Thy Beloved. If you like a very theatrical effect and presence to the music, this is it.
Lastly ill use Power Metal. Easily one of the most popular genres, It emphasizes many of the same things as Symphonic Metal, such as string arrangements, large orchestras, and female and male vocals. But Power metal has a bit more emphasis on the riff and the guitar than Symphonic does. Bands to check out here are Blind Guardian, Stratovarious, Helloween, Hammerfall, Dragonforce, Fates Warning, Metalium, and Primal Fear. Once again, this is even better if you want theatrical effect with lyrical themes that often revolve around fantasy, sci-fi and horror themes.
If you want me to do more genres, simply ask