Kiseijuu, most recently! I watched all the episodes online when I first found it (about twelve, in one day), then was so impatient that I spent the rest of that night and the next two days reading through the whole old manga. And I haven't really watched more of the anime since then, since I spoiled it for myself, but I might pick it back up to see it in shiny-new-action.
I also was pretty obsessed with Tokyo Ghoul for a week or so... I watched the whole first season days before the second one was released... of course, I didn't know it was going to be released (or if it existed) so I read the whole first season-equivalent of the manga before going back and finding out there was more to watch. But... I kind of liked the manga better, so I don't really care about the anime anymore, as much...
About a year ago, it was Shiki and Serial Experiments Lain, but I painstakingly meted them out so I wouldn't run out of episodes too quickly.
And a little while ago, I went from an episode every six months of Neon Genesis Evangelion (the first few episodes are SO SLOW and BORING) to marathoning the last twenty of them in two nights just to see what everyone's always on about. But that was it. I don't want to read the manga, don't care about the movies, honestly, it was such a waste of time (except for the bitchin' theme song which I now know by heart). But I did seriously marathon it, so it... kind of counts?
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