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Start with the clove hitch, the taut line hitch, and the several ways to use a figure 8. The prussik, the alpine loop, the marlinspike hitch, and the fisherman's knot are all useful specialty knots. Learn the bowline and the sheet bend because everybody will yell at you if you don't. Trucker's hitches are just a combination of previously mentioned knots and are great for strapping stuff to packs or whatever.
From there, I'd say just go crazy. Spend a few hours on wikipedia with some pieces of rope (not string or cord) fooling around. Every knot was invented for a specific purpose, often a -very- specific purpose, many of them common outdoor situations. And it's fun.
