So yeah there really isn't any difference between all three other than Manga being Japanese comics and Graphic Novels being the words people who are too ashamed to admit that what they're reading is aimed at the same demographic as comics and manga are and refuse to accept the medium for what it is. Either way that's besides the point as there are obviously top notch comics and manga from both sides of the spectrum such Watchmen, Sandman, Phoenix, Nausicaa, Vagabond, Monster, etc.
One in particular that stood out is Berserk, I'm still reading it and it has horrible hiatuses both because the author is a lazy bastard and his publishers have horrible schedules which makes it seem like the author likes to take long breaks. To put it in perspective this series has been ongoing since 1990 and shows no signs of stopping, hell it probably isn't even halfway done as the author said he has a lot more chapters he'd like to pump out before the series' closure.
That however is probably the only glaring negative that this series has as it has gorgeous art and a great story, it also remains one of the most violent and disturbing manga out there without letting it overtake the manga and becoming a detraction. Hell the violence while plenty isn't gratuitous and is needed, only shallow fans follow it because of the violence alone, it's got a riveting story with fleshed out characters that you can't help but like and hate. Even the main antagonist who is possibly the most evil character ever in a story of any medium, can't help but be liked for how charismatic he is. Yeah I know I didn't give a good synopsis of it, but I'd rather not spoil it.
Oh and the Golden Age arc which is the main character's backstory has one of the best character developement in the medium, lot's of people tend to skip over the first three chapters and right into that arc. You won't miss much as the story starts in medias res, and you can easily read back those chapters after a certain point in the story. (You'll know once you get there)
This video give a pretty good indication of what the series is like, however it's very spoilerific and may give the false impression that it's nothing but gorn if you don't understand the context for the scenes. The song also fits the whole story and themes that it has.