Open with a specific reference to where and how you met: the booth, the panel session, the lunch table, the conversation by the coffee station. This is what separates a useful follow-up from a mass-send that reads like a lead generation campaign.
"We spoke briefly after the [Panel Name] session at [Event Name]. You mentioned your team was expanding into [market] and I work with companies making that move. Would a quick call next week make sense?" If they remember the conversation, that's enough to get a reply.
If you can't remember the specifics (which happens after a busy conference day) check their LinkedIn profile before writing. Their current role, a recent post, or their company's latest news will usually surface a relevant angle. "We connected briefly at [Event Name] during the [Session] panel. I've been thinking about the discussion on [topic] since then." What you want to avoid is sending a note with no specific detail that could have gone to anyone in the room.