Short answer: there isn’t one single cheapest carrier all the time. As of Sep 15, 2025, low‑cost/secondary carriers — especially flydubai and FitsAir (and occasionally IndiGo on some date combos) — typically offer the lowest base fares on Colombo (CMB) → Dubai (DXB), while full‑service carriers (Emirates, SriLankan, Etihad) are usually pricier except when they run promos. (flydubai.com)

Quick details and how to get the cheapest fare

• Common cheapest airlines: flydubai (regularly shows low fares on this route). (flydubai.com)
• Other budget/discount options that often beat the majors: FitsAir and, for some travel dates, IndiGo. (pk.trip.com)
• Full‑service carriers (Emirates, SriLankan) usually cost more but can be cheapest during sales — always compare. (google.com)

Practical tips

• Prices vary a lot by travel dates, how far ahead you book, and baggage/seat choices — compare flexible dates. (Use Google Flights, Skyscanner, Kayak or the airline sites.) (google.com)
• Check the airline’s website after finding a low fare on an aggregator — sometimes direct booking is cheaper or gives better change/refund rules. (flydubai.com)

If you want, tell me your travel dates and whether you need checked baggage or carry‑on only and I’ll look up live fares and give the absolute cheapest option for those dates.