Joe would be a better one to ask about folders, he has used more, especially customs and thus could make better comparisons, but anyway based on what I have used there are a few real standouts when I think of working folders :
Spyderco Military / Paramilitary / Manix :
The Manix works better for heavy powercuts due to the more hand filling grip, and the other two are better for precision cuts in side and pinch grips. I like the Military over the Para in all areas outside of the lock.
Swamp Rat Rat Trap :
Similar to the Military in a lot of ways, mainly an issue of point style and handle preference. Too bad it isn't a compression lock.
Spyderco Delica / Endura :
These don't offer the same NIB cutting ability, with the sabre vs high flat grinds, but are a lot less expensive. The FRN looks cheap to many, but outside of possible long term issues with blade play in heavy use, doesn't have a real significant downside. They are generally more suited for heavy work, more towards prying and general use than the optimal cutters like the Military with more steel left in the blade and point.
Spyderco Calypso Jr :
Essentially an upscale version of the Delica, or a really small Military. A nice small precision cutting blade. The new ZDP-189 should be interesting.
U2 :
Really nice small folder for precision work, nice steel, efficient grind.
Opinel :
Nothing needs to be said here.
Benchmade 710HSS Axis :
Easily in competiton for the top hard working folder of its price range, and well above. I have used the mini version, the only real reason I have not bought the full one is the abundance of existing quality folders, plus it does irritate me that you have to reprofile them NIB.
Possible upgrades to the above knives :
+S30V rehardening by Wilson to 60/62 HRC
+high hollow grind by Krein to really focus on a pure cutter
There are other decent ones I have seen but would not recommend due to QC issues. The Kershaw Vapor I have is really nice in many respects, but reports of lock issues are common, and the steel in this one is problematic.
The production Dozier's look very solid as well, and for really high end R.J. Martin's should make excellent users. I have not used them, or actually any of R.J.'s work, but have talked to him many times and he really is an upfront no hype guy.
The Sebenza is a solid knife, take care about the promotion as a "tough working knife" because the grind on the one I have doesn't suit that at all. It would be heavy competition for the 710HSS, Paramilitary and Rat Trap at a similar price.
-Cliff