I would never choose only one region as the best, but having lived in the PNW, that gets my vote as one of the few best places.
The year-round paddling, plethora of saltwater launch sites as well as some lakes, solid maritime infrastructure (radio, nav aids, USCG presence, and overall sea-knowledgeable culture, marine supply stores), put it far above most places in the US for sea kayaking.
The fact that theoretically someone could paddle all the way across the Pacific also gives it a certain lustre that nothing inland can match. It’s not just about the distance (the Missouri River is long, for example). It’s about the open water environment, and the tidal elements, and the marine life. There’s a boundless world of life UNDER your kayak out there.
Having said that, now that I paddle a surf ski, the rare combination of environmental/weather factors in the Columbia River Gorge make me want to paddle in the famed section that is mecca to wind surfers et al. So Magooch, I’ll be heading out that way, probably in the new year.