Fullmetal Alchemist is the best one, because it manages to both have the spirit of most well-known manga (i.e., an underpowered, honorable, usually teenager, usually spiky-haired person uses his amazing talent at a specific skill and the power of friendship to defeat evil in a realistic fantasy world) while at the same time sidestepping most of the negative clichés and giving a storyline that's actually interesting, as well as characters that are well-rounded and you can actually care for. It's an excellent one for starting off.
Everyone says wonders about Death Note, but it's painfullly obvious the author was winging it as he went on, which is not how you should do a mystery story at all. Still, the concept is interesting enough to warrant a read. It's quite short, at least (well, relatively so).
Bleach is good in the beginning, when it doesn't take itself so seriously, then it devolves into Drangonball Z WITH SWORDS! [http://tvtropes.org/pmwiki/pmwiki.php/Main/ptitlekt6mtovm4vne?from=Main.INSPACE] with more characters than every other manga put together. Seriously, it's easier to keep track of the characters in One Hundred Years of Solitude than in the later sagas of Bleach.
Hellsing is Bleach except it's even more over nine thousand, and it never stops not taking itself seriously (as far as I know, at least).
I did think xXxholic was about porn addition as well (one of the side effects of being exposed to manga series first on 4chan - you never know when something actually is porn). I've only read the first issue, where it's agressively estabilished that it takes place in the same canon as Sakura Card Captors. I'm not sure what to make of it yet.
Never read Akira or Cowboy Bebop, but I've heard they're, like, genre-defining.
There are a few others I like, but it's down to personal taste. I never heard of any of those other ones that were mentioned here, so, since I am the center of the Universe, I must conclude that they suck.