1st choices:
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Benchmade 710 (4") or 705 (3") Axis lock
Benchmade Pinnacle 750 (3.6")
Spyderco Starmate (3.75")
Benchmade AFCK 800 (4") or 812 (3")
Spyderco Wegner (3.6")
The Wegner is really designed to be a very versatile utility knife, one that happens to be useful for hunters, but any good quality, ergonomic folder with a good lockup like the Wegner is fine for defense purposes.
Spyderco's come very sharp out of the box. Benchmade is sloppy...some are very sharp, most are only marginally sharp. You'll want to learn to sharpen very soon if you use the knife anyway, sooner with Benchmade. Search for sharpening recommendations with search engine.
Use the search engines to locate threads that contain multiple of the knife words recommended in this thread and you'll find a wealth...this question pops up about every other week it seems, which means there are a bunch of similar threads.
If you really want a defense knife, you need to be able to deploy in a reasonably quick manner. This argues for pocket clip carry, but if you can't stand that, a thumb stud/thumb hole/thumb disk is a necessary feature, and a good solid lockup as well.
I really think the BM 710 and 750 are standouts that meet those criteria very well, and both have recurved rear portions in the blade, which can make them a bit harder to sharpen, but it also makes them both very good slashers within the confines of good utility knives.
If you are new to knives, I wouldn't bother with the subtle claimed performance advantages of M2 carbon tool steel over ATS-34. Just get ATS-34 and enjoy the corrosion resistance. Then you are free to choose Black T coated blades or just plain.
If you are new to knives, don't buy out of a catalog until you've handled a bunch of knives locally (if you can't get a good price locally). You may see a lock that looks simple and easy on paper, but that in fact can't be easily manipulated with one hand.
I would guess that most of us that have used the Axis Lock have found it to be more secure for hard work and for defense purposes than a liner lock in a general sense. There are great liner lockups that are recessed, but you are less likely to encounter lock failure probably with Axis or REKAT's Rolling Lock. I wouldn't worry about "pushing the button" on the Axis. Note that a stabbing thrust motion tends to cause the Axis button to move tighter into the locked direction. A clenched grip tends to have your full hand behind the axis button. And under adrenaline rush, you'll have a clenched fist all right.
[This message has been edited by rdangerer (edited 03-30-2001).]