I need to repair a ripped seam on my large rucksack by hand. The repair should be strong enough to hold around 20-25kg. What thread is appropriate, and what kind of needle and stitch should be used?
It depends on the material. For heavy canvas, or leather reinforcements, a hand-awl that stitches like a sewing machine is useful. These awls usually come with an assortment of needles and some waxed thread. N.B. The needles are brittle; do not exert sideways force on them.
For nylon and other lightweight fabric, medium nylon or carpet thread can be used with any sewing needle with large-enough eye. The important thing is to use enough stitches to cover any area likely to pull loose. Be sure to overcoat the stitched area with a sealant, both for waterproofing and to reduce likelihood of stitches pulling out. Though there are commercial seam sealants, I've used urethane or vinyl cements successfully.
If it's an area likely to be abraded, e.g. bottom of pack, a bit of adhesive tent-patch tape or iron-on tape, carefully applied (don't melt a nylon pack!) makes the repair last longer.
I would visit a shoe repair shop for a length of heavy thread for the job at hand. The cobbler would be a good person to consult for advice on the gauge and composition.
I would recommend a baseball stitch that you find along the seams of a leather-covered hard or soft baseball. The benefit of the stitch is that it pulls the edges together in a butt-splice that remains flat. The distance on either side of the seam can be varied to accommodate weakened fabric. The stitch also reduces the edges unravelling.