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Biased recommendations: Fairy Tail and Noragami. The former's long but is pretty enjoyable and has a good cast and fairly good plot (it's not a super good one, though XD); the latter is also fairly enjoyable and has a good cast and plot.
Less biased recs.: If you're into comedy, SKET Dance is a good read; it's a really good mood lifter, too! (Given, there are some sad parts.) It's relatively long (nearly 300 chapters), but it's completed, so that means no more new chapters XD Rave Master is also a good read also a somewhat biased opinion with fun characters, story, and so forth. The art isn't the best at the beginning, but it gets better later on. And if you like shoujo, Ao Haru Ride's a nice read.
EDIT: Adding on from earlier, I thought of a few supernatural manga I found enjoyable: Shaman King, Mushishi, Jigoku Shoujo/Hell Girl, and Natsume Yuujinchou~