Spyderco urban for me if we are talking legal carry. Much more descreet than the UKPK and VG10 steel.
For a locker it's the benchmade mini grip. It was a waste of money getting the one with the black coating, but the steel is great and the knife feels indestructible.
Not my favourite, but a good one hand opener is my victorinox cadet.
It's on a mini S-carabiner with the lanyard attached to my belt. So if I only grip the blade the tension on the lanyard pulls the knife open. I'm not sure it's totally safe but I've never had any accidents doing it.
Same with the spyderco grasshopper. When attached to a set of keys it can be opened with one hand.