If you're only going to have ONE knife, the 7 1/2" blade Bowie-style is not a bad choice. Big enough to use for serious chores and small enough to do more delicate tasks, and the Bowie-style blade is the do-it-all design. It doesn't do much the best, but it'll do just about anything. The Ka-Bar and Ontario copies of it are good knives now for the same reason they were when they were first issued: They are strong because, while the steel isn't the hardest, is IS thick. They don't hold an edge great, but are easy to sharpen to a keen edge quickly in the field, and they are cheap enough to simply replace. The USMC fighting knife is 7 1/2" long blade, the shorter version, I believe is a 5" blade. If you're on a budget, this is a good place to start.
If I had to pick my one, favorite knife, it would be the Swamp Rat Knifeworks BattleRat:
Swamp Rat Knife Works
Battle Rat - $148.95
Patterned after the infamous Busse Combat Battle Mistress, the Battle Rat is the flagship of the Swamp Rat line. For those who like everything bigger, better, and badder... this is the only choice for you.
This knife is the epitome of "rat resilience". Known as "fierce survivors", rats are the most resilient mammals on the planet. In order to survive, they will squeeze through a hole the size of a quarter, scale brick walls, swim for over a half a mile and then tread water for three days, gnaw through lead pipes, cinder blocks, and glass using chisel-like teeth that exert an unbelievable pressure of 24,000 pounds per square inch, withstand being flushed down a toilet, enter a home using the same route, and even plummet five stories to the ground without a scratch. They have adapted to not only survive, but thrive in unsurvivable situations... including surviving atomic blasts. After WWII, the U.S. carried out atomic bomb testing in the Enjwetok Atoll. Enormous craters were formed, the vegetation was eliminated, and the atolls were engulfed with giant waves. Each blast spread lethal radiation beneath the mushroom clouds. The small islet of Engebi was one of the worst hit. On this islet lived our hero... the rat. A couple years after the holocaust, biologists returned to Engebi. Everything they found, plant life, fish, soil, etc., contained radioactive substances. It was assumed that all animal life had perished. Yet, somehow, the island abounded with rats! They were not maimed or genetically deformed. They had actually adapted to their new environment and, at the same time, increased their life spans beyond what is considered normal for a rat.
The Battle Rat knife possesses these same characteristics. Nothing can kill it. It will withstand falls, gnaw through cinder blocks, and survive nuclear blasts. This big, 9-1/2" long, wide-bodied Bowie is setting performance standards that blades costing three times as much can only dream of achieving. The ultra wide blade offers unbelievable cutting and slicing ability, while the differentially tempered blade delivers untouchable toughness and durability. The shock absorbing Resiprene C handle will absorb the majority of the impact and will allow for extended use with less fatigue. When you need the ultimate survival knife, the Battle Rat will prove to be the ultimate in angry steel.
They aren't kidding, they had video of a guy chopping through concrete blocks with it. The edge was dulled, but not cracked, chipped or broken, and returned to a keen edge with a few swipes of a hone. Then there was the "vise test" The tip of the blade was put in a vice, pipe over the handle, and the knife was bent down to the table the vise was bolted to. NO plastic deformation of the blade.
The only "knife" I've met that can match this for pure "badass" is the Himalayan Imports Ang Kola (sp?). I've got the big 20" model. While it won't chop a concrete block without damage (I assume, I'm not trying it), or bend 90 degrees in a vise, it will take a tree down faster than the Rat. It isn't much good for small jobs, but does come with a smaller knife and hone in the sheath. So if you consider the Kukuri, the smaller knife and hone (they all come in the same sheath) as one knife, this is it. It's replaced my BattleRat, Machete and hand axe in my kit.
It's also really easy to sharpen and it's been about 4 years since I last sharpened it, and it's just starting to dull. 
