There is no definitive answer to this question as the type and size of shark you are hoping to catch will dictate the line strength you need. For smaller sharks, such as spinner or blacktip sharks, 20-30 pound test line should be sufficient. Larger sharks, like tiger sharks or great whites, will require a much heavier line in the 80-100+ pound range. The size of your reel and rod will also play a role in how much line you can effectively use - remember that you'll need enough line to make multiple casts and have some left over in case of a long fight. Ultimately, it's best to err on the side of caution and go with a stronger line than you think you'll need.