If I had to pick one folder ( subject to my previous caveat ) it would be my custom Crawford carnivore with rolling lock. The REKAT production folders used the same lock and although lockup seems solid it's not very smooth or as reliable as implemented by them. Honourable mention to the sebenza frame locks. If balisongs were legal that's a brilliant design with the benefits of a fixed blade and a folder, but of course that makes too much sense to be legal.