Not surprisinginly, there's really not that many comics that have successfully reached that number.
Of course I haven't taken into account certain issues that may have reached that number had they not been re-numbered.
Nope. I'm only counting those runs that officially hit issue 666.
So without furhter ado, here's a brief rundown on those titles that did hit 666, and what they were about.
-Action Comics#666 (June '91):
Basically Superman comes across red, alien crystals on the moon, and from there, shit goes cray. Very, very cray.
In the previous earlier parts of the story, Supes contact with said red alien crystals make him turn extremely paranoid and he flips out, to the point that he accidently kills Lois Lane, Jimmy Olsen, Booster Gold, and all of Metropolis.
He's then confronted by Guy Gardner and the Martian Manhunter from the JL, but he kills them too.
The United States then declares Superman a criminal threat to humanity( yeah, no shit sherlock) and sends in the big guns after him.
Said big guns are Firestorm and Captain Atom.....
......which does NOT turn out well for them at all.
After that, the Doom Patrol (In a very rare appearence since they belonged to the Vertigoverse at the time) show up only for Superman to scream them to death. No, really, he does.
The WW shows up with a piece of Kryptonite so that Supes can kill himself.(Why WW? Was Batman too busy?)
Writer Chuck Dixon didn't chose to exploit the 666 theme, but in a sense did deal with a "devil" in the man called Bane.
Coming out in near the end of the Knightfall storyline, Detective Comics#666 mainly deals with the new replacement Batman, Jean Paul Valley, looking for Bane.
Commissioner's Gorden's men captured Bane's associates Trogg, Zombie, and Bird, but after refusing to give up Bane's whereabouts, taken back to their cells.
They find a note, they think from bane, letting them know they're about to be sprung.
They are, but its because of AzBats, not Bane. He follows them back to Bane, and then shit gets real