With this one I am truly torn between three manga...(and this is only completed manga I have managed to finish lol)
Dragonball - I have trouble comparing my love of this manga to others because I have liked it since '97 (first saw the anime ) and it brought me into manga and anime. Even rereading it is fun, but sometimes I wonder if its just because it reminds me so much of a younger me and time that I still like it as much as I do, and if I would still like it this much if I just started it now...
Slam Dunk - This would be the manga that brought me over to the sports side of manga. Amazing manga in my opinion (and almost everyone else's on this thread it seems lol) and probably one of the best sports manga ever written.
Chrono Crusade - I'm not sure how well known this manga is, but I think a pretty decent amount of people have heard of it. This was the first "dark" manga I ever read. Short at 8 volumes but a magnificent series. I was addicted waiting for each volume to come out. Still a great read when I'm bored. I recommend it to anyone who hasn't read it.
Like others responding in this thread, I seem to have a number of manga that I've been fond of over the years. A lot of it comes from exceptionally good mangaka such as Yoshikazu Yasuhiko (Mobile Suit Gundam: the ORIGIN), while others have a particular quirk that makes them somewhat enjoyable in spite of various faults.
Mai the Psychic Girl - drawn by Ryoichi Ikegami, this was a nice short little manga story (27 chapters) that helped wean me off of my X-Men addiction during the late 1980s and showed me that a heroine does not have to have an enormous bustline or a mega-gonzo attitude towards fights (Mai wanted to avoid them, if possible) to strut her stuff.
Akira - Katsuhiro Otomo, out-of-control genetically engineered psychics versus biker punks and shadowy military officers, 'nuff said.
Venus Wars - anime character designer Yasuhiko threw his hat into the manga creator's ring with this series, winning me over to his simple, yet excellent storytelling and design style.
Caravan Kidd - Johji Manabe before he went into his hentai kick produced a fun series about a runaway android warrior and her con artist companions.