It's hard to give recommendations because the stuff I read is probably not what most people would like ;-) Especially, I tend to like sports manga which is pretty far from fantasy. (Currently Yowamushi Pedal is fuelling my cycling addiction!)

I mentioned Barefoot Gen, which is a very early manga and a classic that is actually taught in Japanese schools. It's not fantasy, though. It tells the story of the author's experiences around the atomic bombing of Hiroshima.

For fantasy things, Full metal alchemist is an alternate-earth scenario with magic. It is extremely good. One thing that doesn't actually fit the epic story format because it is a series of short stories is Mushishi. I highly recommend it, although I've only watched the anime. Akira is a cyber-punk, distopian future manga which is very highly acclaimed. Personally, it is not my taste, but it is excellent. Another interesting manga which has no actual heroes is Death Note. It's another one I don't like because all of the characters are really nasty, but some people really, really like it for that reason. I asked my wife (who is Japanese) for a suggestion and she mentioned Ginga Eiyuu Densetsu (Legend of the Galactic Heroes). I haven't read it but Wikipedia indicates it was originally a series of novels. It seems Viz publishes both the novels and manga in English. Ghost in the Shell is a very good sci-fi manga, but I have to admit that it is years since I looked at it, so I can't really remember it.

One manga which I love is actually a baseball manga called Touch. If you read that, you will see the manga story structure very easily.

I started reading manga as a means to learn Japanese. At the time I had only a passing interest in graphic novels and comics. Over time I got quite hooked on it. But I only read in Japanese, so I tend to read things which are quite easy. There are very complex manga stories which are aimed at adults, but unfortunately I haven't spent much time looking at those things.

Of course the 2 current most popular mangas aimed at young boys is Naruto and One Piece. Naruto just finished. It has some very powerful moments and the story is quite good. I've found that there are quite a lot of useful ideas that you can take away from the story and apply to your own life. However, the characters are one-dimensional (on purpose, I think) so I suspect it wouldn't appeal to the original poster. One Piece is also quite good (the first chapter is actually really worth reading on it's own and can stand by itself even if you don't read anything else). Again, it is aimed at a young audience and the characters are very static. There are some extremely moving moments, though.

Hope that helps a bit. As I said, it's hard to give recommendations because some people will look at that list and probably absolutely hate some of the things on there. There is an unbelievably massive amount of manga being written and I truly believe there is something for every taste. However, the examples above are pretty easy to find in English and are all quite famous. If they don't meet your taste, I hope you'll explore a bit further on your own.