The easiest to suppress out of the box, is the P22. Threaded adapters are easy to find and relatively cheap.
There are many options for pistols with threaded barrels, and some don't even require an adapter, so i would disagree with this. Ruger 22/45 Lite comes to mind.
NRA Certified Pistol Instructor & RSO, IDPA Sharpshooter, Project Appleseed Rifleman
Member NRA, GOA, SAF, VCDL