All the obvious ones seem to have been mentioned. If I wanted to recommend you one thing to get first, it would be Watchmen. Alan Moore is almost certainly the best writer in the comics industry, simply because he has the canniness to exploit the form to do things that can't be done in films or novels. The examples I always pull out are the 'comic within a comic' in Watchmen, where panels of the pirate comic the boy is reading are intercut with the panels of the story, and the television studio break-in in V For Vendetta, with the simultaneous lines of dialogue from the multiple television monitors. You really have to read it to understand.
Otherwise, it seems to me that people have been kind of reticent with non-western suggestions. If there's any manga that you NEED to read, it's probably Akira, by Katsuhiro Otomo. This is a pity, because Akira is absolutely fucking enormous: it has to be one of the longest continous graphic novel by a single author/artist behind Cerebus (and you can quibble about Cerebus because a lot of the artwork was a collaboration). It's expensive as well. Each of the six books set me back £20 (for Volumes 1-3) or £25 (for Volumes 4-6), about four years ago. Might be cheaper now, that was before manga really blew up. It's a lot better than the film, with deeper characters, and miraculously it actually makes sense. At the end of the day though, it's probably worth it just for the artwork. It's also a great first time manga, because it doesn't employ any of the Japanese comics conventions that might alienate a reader in other manga (No chibi, no nosebleeds, etc.) Unfortunately, as far as I'm aware, the english translation is only available flopped (ie mirror imaged so it reads left to right, rather than right to left like a proper manga), and all the onomatopeia and almost all the 'in world' writing has been meticulously stripped out by some poor bastard with Photoshop and replaced with english writing. Admittedly, this has been done very proffessionally, but it is jarringly noticeable now and then (for example, when the Americans show up and Otomo had english writing originally on their planes, it is all backwards) In a similiar vein, Masamune Shirow's 'Ghost in the Shell' is a good aquisition. It's been much less fucked around than Akira, mainly just flopped, and although the plot maybe isn't quite as deep, and the artwork is much more cartoony, in the Japanese sense, it still looks great and reads great, and, refreshingly, Shirow allows you the option of choosing whether to be a complete geek or not by shoving most of the hard sci-fi ephemera into copious notes at the end of the thing, which aren't really necessary to the story. Another definite is 'Barefoot Gen' by Keiji Nakazawa, a moving autobiographical account of the atom-bombing of Hiroshima.
After that, you're probably, in all honesty with myself, in the territory where you have to actually like manga and its conventions, though I'd say you'd be pretty safe with Blade of the Immortal and Lone Wolf and Cub, because, hey, who doesn't like mysterious ronin beating the shit out of everyone they meet? And, judging by your selection of favourites, I probably shouldn't go in to any of the extreme stuff, though I think everyone needs to read at least one Suehiro Maruo book before they die, preferably after having taken some potent hallucinogens.
