Well what have you read so far?
If your looking for something like Berserk (which I would recomend anyway ) you could try Hellsing by Dark Horse. It's basically a modern take on the classic Vampire Story (Dracula) which follows an organisation called Hellsing who fights against Supernatural forces; specifically Vampires; in Britain. The twist is that Hellsing use an Ancient and Powerful Vampire called Alucard to hunt his own kind. Can't say much else as it would spoil the Story.
As for Thought Provoking there's Deatenote. It's a Crime/Horror Manga which follows a Student called Light/Kira who find the Notebook of a Death God (Shinigami) and uses it to kill criminals and right the wrongs of society. There is then a second character called L who is working against Light/Kira to try and catch him. The main thought process behind this is whether you believe Light is correct that by becoming a killer himself he can grant Justice or whether L is right in that Light/Kira is nothing more than a killer himself therefore any justice promoted in this way is Hypocritical?