Aoi Hana which is a legitimate wish that it gets released faster (a lot of the people complaining are doing so for weekly manga... how much faster do you want it released...?). It's bimonthly meaning that it comes out once every two months, and so far, the story has reached a sort of climax that really has you waiting with jitters for two entire months to continue.
Theres a bunch of others as well, and I have posted awhile back, but recently I have really been wanting to see more of Sekirei. Maybe it's due to the fact that the second season is currently airing, and reminding me of the series, not sure.
I would say xxxHolic, I would love to get chapters more regular than just one per month. Two chapters per month would be great, but ofcourse it's great that CLAMP make the chapters lengthy with about 40+ pages.
But since I'm quite happy with the releases of 'Holics chapters I'll go with Legal Drug, I've been waiting SOOO long to read more, so much that it's getting to the point were I can't stand it. Clamp are so cruel..
20th Century Boys as Viz is only releasing a volume every 2 months... And with like 16 volumes to go, that's 32 months which is 2.6 years before I can finish the series... -_- Which is why I resorted to buying them straight from Japan in Japanese. (And no! I won't consider fansubs, I will translate them myself :) )