I can't say as how I've owned any CS folders, but I've got a couple of their knives, most recently the Peace Keeper I fixed blade, and it's a very nice knife. I'm not a big fan of AUS8, and their knives tend to be a bit large for my tastes, but they do make a good product. I mainly just don't like the prices and designs all that much, but if you do, you'll probably be happy.
Kershaw--hard to go wrong here. I own a few Kershaw, and will be picking up a ZT when I have the money for one. They do make excellent knives in a wide variety of prices, and they have excellent CS. That being said, I don't EDC Kershaw. Maybe I will EDC a ZT when I get one, but none of their knives have really done it for me enough to kick out my favorites out of my pocket.
Spyderco--I used to hate the way they looked, but they grow on you, especially after you use one. Spyderco makes some of the best EDC blades out there, and my FFG Stag Arms Manix 2 in S30V is one of the two knives I regularly EDC. Para2 and Mili are very popular, as is the Endura. If you're gonna get a Spyderco, although the Tenacious is a great starter, I'd really recommend getting one of the really nice models, because truth be told, you're going to get one eventually. Might as well save yourself the effort and just go straight for what you really want.
Benchmade--I'm pretty much a fanboy. The Ritter Griptilian is the other knife I regularly EDC, although I'm planning on getting new handle scales for it, because I don't like the Noryl GTX. They have a wide variety of blades; if you think they're expensive, you're probably looking at Gold class, because most of their knives tend to be quite affordable. The one thing I will note is that their 154CM has not impressed me. I'd go for another material. Their HK line is also excellent, and there's a lot of really great knives. I tend to like BM's designs the most of any of the companies mentioned, but it's hard to go wrong with any of them.