These characters are, in my opinion, the coolest of the badass dudes created by four of the most successful comic book publishers. I feel this way about them mostly because these guys appeal to my taste in character & storyline when I want to read a comic that has a darkness to it you can't get in most stories from that medium.
They may all be similar in ways of having dark qualities about them but at the same time, they are all quite different. I would def classify each of them as 'Antiheroes' but their stories show the different paths one can take to do good but not be all squeaky-sunshine about it.
If you've never read a Spawn, Darkness, Batman or Wolverine book & this piece has you wondering what these guys are about, I def reccomend picking up one of their stories just to see some of the best of comics that come with a darker edge.