One is the hole. I infinitely prefer the hole to any other way to open a knife. The hole is pure genius. It simply works far far better than a stud. Case closed.
Two is the sharpening. Spyderco takes pride in sharpening their knives and they do it properly. Out of the box, Spyderco knives tend to blow me away. Many (most) other makers are shockingly sloppy about sharpening their otherwise superb knives. Don't ask me why. A sharp edge is what a knife is all about.
Three is the attitude. I am always amazed when Sal from Spyderco contributes to forum discussions. He and his son Eric clearly love knives. For them it goes beyond making money.
My favorite Spyderco is the Manix. The Paramilitary is a close second.
Knifeworks is where I got one Spyderco Sprint (a military with CTS204P).
BladeHQ supplied my next via a waiting list (the Manix-2 CF with 154CM and S90V).
Tom's Knife Info / email@example.com