To survive you need food, shelter and water, this dosent necesarily include a rambo sized "survival knife".
Fire makeing- Hatchet, hands down
Shelter makeing-Hatchet Hands down
Add a drop point skinning knife and a small capeing knife
Trust me, I've spent enough time outside to know what I'm talking about.
If you plan on surviving for long,
cord, metal cup/pot for cooking, boiling to sterilize water
water purification tablets
warm cloths/rain gear, appropiate for location
food and/ or gun
topo map/ compass
matches/lighter, dry tinder, its VERY difficult to make a fire without these
tarp/tent, unless your certain you can create a weather worthy shelter to keep yourself warm and dry with only a hatchet and knife
I peronaly use a gerber hatchet, with capeing knife in the handle. I also have a gerber gater, drop point, half serated. Its made from 154cm steel. Please read these articles before you make your selection:
http://www.agrussell.com/knife_information/knife_encyclopedia/articles.html I also recomend a book called "Complete Guide to Hunting knives" It covers axes, saws, knives, field dressing, blade design, and steel choice in great detail and accuracy.
Happy Backpacking