I think it depends on your interests; in no particular order... I should point out that all of these titles are licensed. I don't usually read scanlations.
*edit* Description, eh? Sounds like a good idea ... I shouldn't have included so many titles.
- Kira, a very shy artsy-type girl, falls in love with Rei, a violent, motorcycle-loving playboy. Romance and Angst-with-a-capital-A ensue.
- Hatsumi Narita goes out to buy her younger sister a pregnancy test and is caught in the act by Ryoki Tachibana, the neighbourhood bully. He blackmails her, demanding that she become his "slave" in exchange for his silence...
- Two young women named Nana - a punk-rocker type and and a girl with lots of boyfriend-troubles - move to Tokyo in order to pursue love and careers.
- Chiaki is a top-notch piano student who'd like to study music overseas, but is afraid of flying. Noda is a student in the potential dropout class who can't even read a music score. Somehow, romance blooms.
: In post-apocalyptic Japan, a young woman named Sarasa must take the place of Tatara, her dead brother, as the "boy of destiny" and save her country from the tyrannical Red King and his family.
- Setsuna, who is hopelessly in love with his younger sister, also happens to be the reincarnation of a fallen angel. Inevitably, he's pulled into the conflict between Heaven and Hell.
- Watanuki is tired of being able to see spirits, a problem that Yuko (a witch who claims to grant people's wishes) says she can fix -- but Watanuki will have to work in her shop to pay off his debt.
-Nausicaa of the Valley of Wind
- An apocalyptic war has covered the earth in a constantly-expanding forest of deadly fungi, and earth's remaning population is fighting over the remaining habitable lands. Nausicaa, a headstrong-princess-type who lives in the "Valley of the Wind" is thrust into one of such conflicts. A classica manga - one of the first I read.
- A slice-of-life~ish hard-sci-fi tale revolving around space debris collectors, particularly Hachimaki, who dreams of becoming part of the crew of the first manned mission to Jupiter.
- Honestly, who hasn't heard of this one? When an attempt to revive their dead mother with the power of alchemy goes terribly wrong, Edward loses his leg and arm, and Alphonse loses his whle body. Now, the two of them search for a way to restore themselves.
- On the streets of New York, Ash Lynx - a young and attractive gang-leader - stumbles upon a dying man who tells him, cryptically, "Go... see... Banana Fish". Obviously, this has to do with some sort of massive government conspiracy, which will take 19 books to unravel. Hints of shounen-ai, incidentally, though nothing serious as of book 13.
- Light Yagami (or Ratio, apparently, depending on whether you're reading the Viz version or a scanlation) finds a notebook that allows him to kill people simply by writing their names in it. He decides to use this "Death Note" to rid the world of criminals, and quickly finds himself pursued by L, a master detective. A totally un-put-down-able manga so far, seriously...
- A doctor controversially decides to save a boy's life instead of a politician's, only to realize that he may have made a very grave mistake when a trail of mysterious murders leads back to this seemingly innocent child.
- The Hellsing Organization hunts vampires - OMG Nazi vampires - with the help of Alucard, who is, himself, a vampire. A bloody but strangely good read.
(technically a manhwa, but you know...) - A vengeance-seeking priest sells half his soul to the devil to meet his goal... Exceptionally gory right from the start, but the artwork is amazing.
Wasn't there a recommendation sticky-type thread before? It'd be neat to compile a list of manga series with descriptions and such, although some sort of agreement would have to be reached as to which series qualify...