Short answer: Dialog (Dialog Axiata) — independent measurements show it provides the best overall coverage in Sri Lanka. (opensignal.com)

Details and evidence

• Opensignal’s country report (Feb 2025) gives Dialog the highest Coverage Experience (9.0/10), with SLT‑Mobitel second (7.3), and Airtel and Hutch notably lower. (opensignal.com)
• The same report shows operator Availability and 4G availability numbers (e.g., Airtel 4G availability ~92.3%, Dialog ~91.5%, Mobitel ~87.3% in that report) — so “best coverage” depends on the metric you care about (geographic coverage vs. 4G time vs. consistent quality). (opensignal.com)
• Dialog has also been actively rolling out advanced network tech (5G/5.5G trials and lab/commercial demonstrations in 2025), which supports its lead in measured experience. (dialog.lk)

Practical caveats

• Coverage varies by exact location (city vs rural, specific districts, indoor vs outdoor). The “best” network for you may differ depending on the city/region you’ll be in.
• If you need a recommendation for a particular city or route (Colombo, Kandy, a coastal area, hill country, etc.), checking the operators’ coverage maps or the latest Opensignal/Speedtest reports for that area will give the most useful, local answer. (opensignal.com, dialog.lk)

If you want, I can pull the operators’ official coverage maps for the specific area(s) you care about (Colombo, Galle, Nuwara Eliya, Jaffna, etc.) and compare them.