The best survival tools in this day and age remain a wallet with a few hundred bucks in cash (small bills preferred), and a couple of credit cards with significant limits, and a charged cell-phone.
That said, a Swiss-Champ* is all anyone would need for a normal weekend away in the woods. But then, this also applies to many other knives as well. However, if I was thinking along the lines of something for a "survival situation", I'd be considering something large that can be used as a weapon against four (or two, especially) legged predators. My CPK Behemother is strapped to my camping/go bag for a reason.
* Make no mistake, I love this model, and have several of them.