Could you elaborate on "adult oriented"? Do you mean gritty, hard-nosed stuff?
The Dell adaptations of novels are really good stories (Zane Grey), and in general the Four Color books had book length stories so they tended to have more elaborate plots. They also had serviceable art.
As someone else mentioned, DCs had great art. I only have a few of them but the stories are less elaborate but pretty good.
Thanks for all the input so far everyone. I'll have to start making some lists.
JohnT: as far as "adult oriented", I guess I mean something with a decent, believable and somewhat realistic plot. All kinds of western themes are o.k.: law men, outlaws, scouts, settlers, trappers, etc... Doesn't need non-stop action.
I hope that cleared up what I meant some.