What should I tune my guitar to?

So I don't want to keep tuning my guitar up and down but there are always some songs that I can't play when I'm in a certain tuning. Which tuning would get the most songs out of the music I like? The lowest I'll go is Drop C. I have another guitar that stays in standard tuning so I would prefer it not to be in standard. The bands that I like are: Alice In Chains, Amon Amarth, Arch Enemy, Black Sabbath, Blind Guardian, Burzum, Cannibal Corpse, Children Of Bodom, Cradle Of Filth, Darkthrone, Death, Def Leppard, Dethklok, Diamond Head, Dimmu Borgir, Disturbed, Dream Theater, Emperor, Enslaved, Five Finger Death Punch, Graveworm, GWAR, Iron Maiden, Killswitch Engage, Korn, Lamb Of God, Marilyn Manson, Mayhem, Megadeth, Mercyful Fate, Metallica, Nightwish, Obituary, Old Man's Child, Opeth, Ozzy Osbourne, Pantera, Rage Against The Machine, Rammstein, Rob Zombie, Rush, Slayer, Slipknot, Stormlord, System Of A Down, and Venom. What I am looking for is a certain tuning that will allow me to play the most songs by these bands. Please Help!!