I carry a Spyderco Endura4 every day. It ain't pretty, but it fits my hand better than just about any other knife I've ever packed. Of course, like the others have mentioned it's just so easy to take it out of my pocket, flip it open, and close it up without letting go of what I'm cutting. Plus in weather like this I only have to take one glove off to cut a feed sack open!

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I like the spyderco Endura because it won't break the bank, it's tough, ain't chicom, decent steel, and I can flip it open faster than the AO knives I've tried. I always have a traditional on me though, usually a stockman or a Case loomfixer.
I'll disagree with Dave though. If only we could get Spyderco to make a knife with real steel

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A nice CV like 1095, heat treated like the old Western knives. Mmmmm, that'd be a knife!
“There are things in the old Book which I may not be able to explain, but I fully accept it as the infallible word of God, and receive its teachings as inspired by the Holy Spirit.”
Robert E. Lee