I'd lean towards local service businesses-but honestly, the vertical matters less than picking one where you can prove quick wins without burning out.
HVAC and plumbing have worked well for a few agency contacts I know. The leads are high-value (a boiler install can hit £3-5k+), owners actually answer their phones, and you can show ROI fast with local SEO and Google Ads. The trouble? Everyone and their dad is pitching these folks, so your outreach has to cut through.
What I've found is that showing beats telling with trades. Instead of "we do SEO and can help you rank," lead with something concrete-like a quick audit video that points out exactly where they're losing leads to competitors. They get ten template emails a day already, a personalised video stands out.
Automotive repair could also work if you target independent shops rather than chains. Slightly less saturated than HVAC, but same principle applies.
One thing I'd avoid: spreading too thin across multiple niches while testing. Pick one for ninety days, go deep, refine your messaging, then decide.
What's your current outreach method looking like-email, calls, LinkedIn?