Parasyte (the art style took some getting used to, but the story developed really well and came to what felt like a fitting end. I should really watch the films at some point)Ranma 1/2 (okay, it's not that short and it's the same jokes over and over again, but I find it oddly hilarious)Akira (I read the Marvel colour editions originally. Attempting to read it again in black and white in Japanese is going to melt my brain)PlanetesThe Lucifer and Biscuit Hammer (recommended here - why this was never made into an anime will never cease to amaze and sadden me)Zetman (also recommended here. I love the art to bits)Video Girl AiDeath Note (great until volume 7, then it all goes so so wrong)Yotsuba& (it's ongoing but I can live with that as it's awesome)Battle Angel Alita (yeah, the last volume was rushed but it was still an awesome series. I re-read it before mainlining as much of Last Order as I could find. Last Order is....not good)Maison Ikkoku (despite wanting to slap Godai most of the time, and his housemates, it's a remarkably sweet series)Gantz (uh, I still mostly enjoyed it despite the slump in the last third. Also not particularly short)Midori's Days (absolutely insane concept but great)
Aqua + Aria. - Slice of life series about female gondoliers on Mars. Part day to day antics, part contemplations about life and part slightly supernatural. Love it.Yokohama Kaidashi kikou - Another slice of series about androids living in the future where human society has fallen. Really gentle series but it's hard to recommend since there's no obvious hook or continuity.Mahoraba - Yet another slice of life. About a college student living in a kooky apartment run by a high school girl who has multiple personality disorder. Starts off fairly whimsical but it has a touching end that I'm a sucker for,Angel Densetsu - Written by the person behind the Claymore series, this is a comedy slice of life series about a high school guy who looks like a yakuza/sterotypical villain but is actually a nice person.[Continuing series]A Centaur's Life - Yet another slice of life series about a modern day world where humans don't exist and focuses on centaurs and various other supernatural beings instead.
Monster (hands down my favorite Seinen manga)Blame! (this is better than Alita imho!)And these: Rurouni Kenshin, Touch, Ashita no Joe, Bakuman, Breaker.I've just got up-to-date with Kingdom, it's about the forming of the first Chinese empire. Really good series, I can recommend it if you like history and fights. Although they killed off my favorite character pretty early, which sucks.
My own recommendations:
Eyeshield 21 - Great, funny, American Football series
Onepunch-Man - Comedy superhero series
Hikaru no Go - Sports maga about the boardgame Go
Hunter x Hunter (up to the introduction of 'The Ants')
Beelzebub - School life, thugs and the son of the devil in this comedy/fighting manga
Bakuman - A manga about manga creators, it's brilliant.