Black Sabbath is THE metal band. Specially their first era, the gloomy and doomy songs with Ozzy, is the best of the best. Of course, DIO era is good and all, and DIO kicks Ozzy's ass, but you cannot match the raw metal of the first 4 albums. But well, I am a Doom kind of guy.
Judas Priest is my second on this list, but there are far better NWOBHM bands in my opinion (Witchfinder General is my favourite, but then, I voted Ozzy's Sabbath :P ).
I prefer Iron Maiden to Motorhead, but only because I have not listened a lot of it. About Iron Maiden, I really like < 20 songs. I really love those, but their other stuff, for me, is MeH, at best.
Elmofongo: Lemmy did help make Thrash Metal :D
Metallica, Slayer, Anthrax, Megadeth, Exodus, Testament, Overkill, Death Angel, and so much more would be nothing without Motorhead.
And Black Sabbath "made" Lemmy. If you like "Metal theory", check a documentary called
A Headbanger's Journey. I remember himself referencing Black Sabbath's innovation and influence, yet I might be wrong.