I like any of the GTO series.

I can speak for all of those save Dogs and Orion to the effect that they are fantastic. Otherwise, and I assume we're talking about manga available in English:

Hellsing
Excel Saga (much different than the anime and much better as well)
xxxholic
X/1999
MPD Psycho
Bokurano
Blame (out of print, unfortunately, but you can find at least half the series for cheap on Right Stuf)
Biomega
Uzumaki
Drops of God
Yotusba!&

Those are just a few off the top of my head.

Rbx5 said:

Uzumaki

THE CURSE OF THE SPIRAL!!!!!!!!!!

Someone on here pointed out Fourteen once, and I read it all twice. It is so stupid and amazing. Dr. Chicken George is the best character ever created. Daryl Surat was pushing Ressentiment a few years ago, and I believe the second half of it has finally been scanlated, even (the general consensus is that it will never be legitimately published in English, so don't feel bad). I just read Kurosagi Corpse Delivery Service in two days the other week, and I finished Homunculus shortly thereafter. I sincerely recommend reading all of JoJo's from beginning to end. It'll keep you busy for a while. Riki-Oh is required reading. Detroit Metal City. Liar Game. Gamble Fish is horrible, but it has moments in it that will make you laugh out loud because it's so asinine, so maybe it's worth it.

There is one series that I’m really enjoying at the moment, but the manga isn’t technically legally available, but it’s been faithfully adapted into an anime. You can watch that legally, and continue the story by reading the scanlations

The World God Only Knows by Tamiki Wakaki. The basic premise is that the series protagonist, Keima Katsuragi, is basically the greatest dating-sim player ever who can and has beaten every dating-sim gal-game, so he gets tricked into having to help hell capture these escaped souls that hide in the hearts of females. I know the premise sounds kind of interesting with that idea of whether video game skillz can translate to real world skillz (like Scott Pilgrim), but also super anime and you expect it to be one of those really fanserivey shows with most of the story pretty much telegraphed in the early chapters, but it’s not. It's got a great protagonist, good pacing, and a lot of fun.

That is an understandable stance to take after you hear the premise because lord knows anime/manga storytelling does have a fairly similar storytelling format, but I dunno if I agree with that assessment for this series because while it is otaku-y, it never gets really fanservicey with the girls and sexualizing teenage girls to an uncomfortable degree. I don’t think there’s even been a panty shot. Keima purpose for “capturing” the girls is to “cure the gaps in their heart”, which amounts in Keima trying to help them fix their own psychological problems to an extent.
Basically I just really like Keima as a protagonist. He's competent, confident, and not perverted. He's someone I don't mind rooting for. He’s like a mix between Abed from NBC’s Community (complete mastery and understanding of an entertainment, but comically lacking in basic social skills) and Daryl Surat (strict adherence to otaku life and code with seeming disinterest of the real world and its people).

I think it would be more interesting if it were presented as a psychological-horror thing, because I could see an interesting story contrasting "he's so good at getting lonely girls to fall in love with him!" with "but the instant that they do, their souls are consumed by demons!" And he hates himself for doing it, but he can't stop, because he's addicted to the feeling of "girl likes me! Girl wants to talk to me!" And the story could be an ironic twist on the usual "oh this is terrible, all these girls keep falling in love with me" bit.

I just finished watching the first and second season of "God Only Knows" on Crunchyroll and found it pretty entertaining. Great theme songs and they kept it classy, no tentacles or panty shots. Main character is funny but I felt sorry for his parents...
Most of the manga I have been reading on a regular basis was in Shonen Jump magazine and now Shonen Jump Alpha, the online version since its no longer in print. Its a decent service but it could be a little more user friendly.
They began a series called Barrage some time earlier this summer and its interesting, would definitely translate well into anime form.

I have a week off of work coming up and I thought, 'Hey, I should be able to get caught but with jojo's during that time, how long can it be?' Ya, I am going to need more then a week for this.

Ar they making the entire series into an anime or just part of it?

Yay!

Oh yeah, Oyasumi Punpun by Inio Asano is one of the greatest fucking comics I've ever read, so definitely check that out, though alas it hasn't been released here. Viz did put out his graphic novel Solanin, however.