You will receive much good advice here, based upon the practical experiences of others. But do not overlook learning how to use a knife properly. Like anything else, a knife requires maintenance and care. Reading about it is NOT the same as having someone show you how, having you demonstrate what you have learned, and having them correct any errors.
Also, you may need to consider what laws apply to where you are planning on carrying your knife. This is one reason that some have more than one knife.
And also, if you really are planning on using a knife as a weapon, understand the law as it pertains to self defense where you are (and where you might be going to). Expert instruction is required to use a knife effectively as a weapon. Without it (and the mindset to use it) the knife will be taken away from you and used against you. A knife fight is a close-up, intimate, and very messy affair.
Lastly, a knife should "feel right" in your hand, performing whatever tasks that you require of it. You will only know this when you pick it up.
I carry a Spyderco Police model and a Leatherman Supertool everyday, and add a USAF survival knife when I backpack. There is also a small folding knife in my survival kit, but the make and model escapes me at the moment.
Good luck in your search. The journey is half the fun. <img src="/images/graemlins/laugh.gif" alt="" />
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"We are not allowed to stop thinking"