Spyderco Endura. I lost my first one in a snow bank at a bus stop years ago. When the snow finally melted, I went out and found it. It had been buried in salty snow for 2 weeks with city buses running over it, and when I got it home, all I had to do was buff a light coating of rust off of the blade with a Brillo pad. It cleaned up like new.
I still have that one, and keep it in my car. I have a new one I carry every day. Crazy sharp, and the thumb-hole opening is awesome.
Here some options I choose from to accompany me on a daily basis: a CRKT Polkowski Kasper fixed blade, a CRKT Elishewitz Pharaoh auto (2), an S&W HRT, a Buck assisted opening, a Kershaw assisted opening, an S&W mini Magnum auto, a Puma mini, and an S&W Border Guard. Except for my custom Pharaohs, most cost less than $25, but serve me very well.
My current everyday carry is a Smith & Wesson Extreme Ops Rescue knife. It also includes a seatbelt cutter, and a window punch. Many people may find those features hokey, but I am a big advocate of maximizing capabilities. The knife is used primarily for utility purposes rather than defensive tactics. However, I am currently in the market for a good defensive blade.
I have to be honest, my all time favorite knife is still a Stilleto switchblade with a 4 3/4" blade. It's light, concealable, easy to handle, easy to deploy and has an intimidation factor that not many other knifes can match.
Order now and get a second knife free! Just pay shipping and handling.
Anyway, I love the SOG Flash II. I like 'em because they lock closed and the way the clip is designed, when clipped to the edge of my pocket, you just see a bit of the clip - none of the knife shows. You can get aluminum, black, camo, etc... on the handle and the knife, and you can get all straight blade or half straight/half serrated. I also have the Flash II Rescue version, which has a curved tip, so you can slide it along something to cut a strap without cutting someone.
I used to Carry a Boker AK-74 auto, but I lost that into some dark abyss last year.
I wanted another auto-opener, but I became a fan of Cold Steel knives and they don't make any auto's. I now carry a Cold Steel Recon 1, it's a big mama-jamma folder (9.5 inches open). If you open the blade and put it in a vice, you can hang 300 pounds off the end of the handle with no breakage... Mine is partially serrated, not shown in the photo.