Two smoothest folders I've ever owned were a microtech mini-socom elite, and a lg. reg. sebenza. Both of these were/are the the best locking frame locks I've ever experienced. Both also have a pivot brushing.
And by smoothest, I mean, "smoothest with a strong, solid lock up with out play". You can make any decent knife open super duper smooth by over loosening the pivot any time. -Any axis lock, perticularly with a heavier blade can be very smooth too. -No need to go custom imho.