I would also recommend Vagabond, it's in my top five GOAT manga. I have up to volume 11 of the Viz Big editions but I am so behind you wouldn't believe it.
The art is breathtaking, though unfortunately it seems Takehiko Inoue's art has declined in quality just a tad looking at the current chapters. Or it could be the art looks better on paper but it just doesn't have the same depth reading it online. But my gawd, the art in the those earlier volumes will turn your eyeballs inside out.
What's funny is that Inoue planned to finish it towards the end of 2010, but he became sick and took a lengthy break, returning in 2011. The series is supposed to end with a historic climactic duel, though I don't really know where he's going with the story right now. It's likely to finish before Berserk.
If you want to read a manga with the worst irregular chapter releases and hiatuses, ******* has got ya covered. It's actually pretty good, kind of fantasy heavy metal meets dungeons and dragons. But **** me, we're lucky if we get a volume every 3-4 years. Not to mention the mangaka lost tons of his work and then just redrew some of his older content.
I can safely say Berserk will finish before *******, and that's no exaggeration.