Most of mine are easy for an adult to open, but I purposefully stay clear of the large bulky models that have a reputation for hard pulls.
Pony Jack - above medium
Gambler - solid, but smooth and easy
White Owl - easy
Babylon Hill - easy
Ben Hogan - solid but smooth and easy
Fine for an adult, but I thought the pull on the Boy's Knife was much too strong considering the name. No way a pre-teen kid could open it safely. The snap on that thing would likely amputate my 7-year-old's finger.
I think the geometry and leverage has a lot to do with the Gambler, Hogan, and Babylon Hill.