Depends on if you mean "one folder for edc" or "one folder for any use" or "the folder is the only knife you own".
I've been carrying the same knife every day for 6 years now. It is my "one" EDC knife. But I wouldn't choose it if it was going to be my only folder. And the if a folder was going to be my only knife, then that would change things quite a bit as well.
My "one EDC" knife is my Boker Exskellibmoor II. Nothing fancy, just a 2.75in 440c blade. Its legal everywhere near me, and doesn't scare my co-workers in the office. Opens my packages/etc just fine.
Not my image, but this is the knife I carry.
For my only folder? I'd probably go with something like an ESEE Zancudo/RAT 2. Can work in most situations, and doesn't really look out of place. Seems dependable.
If I could only have one knife, and it was a folder? Thats hard. I might go with something like an Alox SAK Farmer though. I like camping, and that saw would help out, and the awl strikes firesteels super well. I'd have to get used to not having a pocket clip style knife for EDC stuff, but I'm sure I'd figure something out.
Alternatively, I like svord peasants a lot. The fact that they're like short handled fixed blades with folding handle extensions means they're pretty robust. They have pretty good geometry and steel, and if you take the time to dress them up a bit, they can look quite nice. Here is my peasant mini that I put some blujean micarta on.