Good quality 9 piece metric screw, bolt and stud extractor kit. No one wants shear off bolts etc, but in the real world they do fail from time to time, for a number of reasons and if you can get them out yourself it's a major bonus.

Extracting screws and bolts, even with the best equipment, isn't child's play and requires a good degree of accuracy drilling the pilot holes. Using cheap 'easy outs' are a very bad idea indeed and the name is an oxymoron! They will invariably snap off and then you're really in the do do, with a visit to an engineer with a spark erosion machine where you will have a reasuringly high bill.

If you're going to extract sheared off threaded fasteners, ensure that you are very careful, particularly with the small sizes. Whilst these are excellent industrial quality, no tool is indestructable if you apply too much torque. Ensure that you drill the remains of the fastener dead centre with a good quality drill bit. It doesn't hurt to pilot drill the holes and open them up a tiny bit at a time. If it is a steel bolt in aluminum alloy heat the area up as much as is safe and use a pentrating oil such as ACF50.