That really depends what you plan to be doing, since the kind of care you'd need to provide can be very different across different situations. The first-aid kit in my car doesn't look much like the one I take hiking: I've got the former kitted out for major trauma (and for minor issues I might encounter away from home) but I don't expect to be using it for long- or even medium-term care of any kind, while the latter spends more space on quality-of-life issues but needs to provide support for the time it takes to hike out.