don't learn just 5. if you need to limit yourself due to time constraints, etc, learn one knot for each application you might encounter. these are the general categories:
loop: bowline
bend (tying two lines together): fisherman's bend or sheet (becket hitch if one loop is fixed such as a splice) bend
shortening (for a rope you don't want to cut): sheepshank
stopper: figure eight
hitch (tying to a fixed object): clove hitch and cleat knot (cleats won't always be available)
learn to splice at least the eye splice, back splice, and short splice, if you have time, you won't regret it.
knots that are adaptable, like the fisherman's (watermans) knot, and the 2 half hitches are valuable. they can be loops, hitches, or bends. i'd avoid reef knots and granny knots because they can be confused with each other.
i've worked on boats for 14 years, and been on them since i was 8, and can say knowing a handful of knots and splices for the 5 main applications is invaluable. be adaptable. one knot might be perfect for application A but be too bulky or not bulky enough for application B. a knot in one material might slip or seize in another, and some knots are harder to untie than others, and thus won't work in every situation.
if your life depends on how well something's tied you'll be glad you took the time to learn them.
best of luck, and have fun learning. don't get discouraged.:thumbup:
caveat: climbing knots are different. i'd consult a book with knots specifically geared to climbing applications if thats where your interest lies.
