Unlined A5 Clairefontaine for journals, Kokuyo B5 Campus Notebooks for study, and whatever is available for loose-leaf or temporary notes (I do have a lined A4 Rhodia pad in case I need to write something good on a single sheet, but I never do).

Perhaps if I got into letter-writing I might be more interested in exotic papers, but I'm quite content with using things that are pretty easy to find and shouldn't be disappearing from sale anytime soon. I also don't use anything larger than a western M, so I don't need super amazing paper for a lot of my general notes.