honest take? No single data vendor handles mixed verticals and geos well. The actual direct-dial accuracy you get in practice - regardless of which shiny logo you're paying for - sits at roughly a third to two-fifths of what they claim in their sales deck.
we ran ZoomInfo for a stretch. phone numbers were better than Apollo in a couple of verticals, but nowhere near consistent, and the price premium didn't pencil out at agency scale.
if you're already in the Clay ecosystem, the waterfall approach is probably the right architecture. layer Apollo + Clearbit + one scraper source, and you'll get materially better coverage than any single provider offers.
one metric worth watching: connection rate versus contact rate. The former is what actually drives cold-call performance, and it varies far more than the database numbers suggest