Well......
If your any thing like me, its going to be impossible for you to decide. That said, I can show you some pics of the knives that have been mentioned.
I personally gravitate to the 5-7" range of blades, I find them to be a good compromise from carrying a knife thats too small, and a knife thats too large, one piece of steel that can handle everything, thats shelter, to preparing fire wood to skinning game if need be.
To start, we'll go cheapest to priciest.
Ka-bar USMC.
I have used this knife alot before I sent it to a friend here on the forums, it was my primary blade for a while, it with a SAK and an axe would cover anything that came along, but most of the time it worked fine with just the sak as a companion.
With a few mods, it can become a great woods knife, but if thats not your thing, I would say it would do you fine as a first fixed blade, ka-bar I think set a standard as to what a fixed blade should provide.
Next would be the BK7, one of my favorite blades, it has seen more field time than any other knife I own. Its solid, can shop well for its size, good slicer for its size and over-all a step-up in terms of over-all ability from the USMC.
6" log of maple took about 5 minutes. One thing about the Becker is that it has a full tang over the USMC which is something I think every Fixed should have.
Never failed getting food cooked for me.
Next up is the RC6, In the end would be my choice of blades out of the three. It chops with less authority than the BK7, but has better slicing capability, feels sturdier, sports a trendier sheath then either other knife, 1095 carbon steel, nothing special but the heat treat is. Plus, it carries the RAT name, which I would trust my life too.
Great knife over-all, and again my man sugestion, if the price is too much, BK7 will be no slouch, just a uglier sheath, and its a little thicker.
Let me know if you have any more questions.