I would rank all three of these companies equally in terms of quality in that $50 sweet spot many companies aim for when producing good quality production knives a notch above the mass-market. The interesting thing about them is that each one approaches their knives differently, and each innovates where the other leaves off.
For example, Benchmade and Kershaw often focus on mechanical precision and clever engineering (Axis lock and Ken Onion Speed Safe). Benchmade pays more attention to blade steel and more traditional "tool" type knives, while Kershaw pays a bit more attention to aethetics and streamlined, compact, assisted opening designs that never cease to amaze.
Spyderco kind of marches to the beat of it's own drummer. Many designs look ungainly, they are not as compact as they could be for the blade, and they generally use the simple and comparitively stiff (albeit tried and true) lockback- no slick assisted openers for these folks. However, Spyderco founded their company on innovation and continue that trend while keeping a "if it ain't broke don't fix it" mentality. First to use H1 rustless steel in a production knife. First to mass produce a waved knife in a consumer price range (And many folks say it actually works better than the Emerson knives that hold the patent). Updates to popular lines that directly incorporate feedback from using customers. And, of course, the use of the excellent VG10 steel in knives of these quality for the price really sets the standard for quality vs. price IMHO.
All three of these companies have something terrific to offer. Just to mention my EDC route in the past several years: Several Kershaws (chives and Leeks mostly), SOG Flash I (clever design, though the fit and finish isn't quite there), Benchmade 530 (thin and light with good steel- just a tad slower to open due to no AO and compact thumb studs), and, of course, Spyderco (Salt I, Cricket in SS, and my EDC for the past 3 months, an Endura 4 Wave). The Endura 4 Wave is definately a bigger knife than any other on the list, but it isn't too big in the pocket, opens even faster than an AO, and really feels like a solid tool in the hand. I've experimented with many options, but I can't imagine many other knives beating it out any time soon.