cassiohui I suspect the answer will depend on what "cheapest" actually means in combination with how many European trips you will take in a year. And how many trips will have a traveling companion (which really would need OW Sapphire for double lounge access)

V class on CX to most European cities (>5000miles) will earn 35 points each way. E class (PEY) would be 45. Since at the moment MPC Silver (target 300) will provide the single traveler with lounge access when traveling on CX it might be easier to aim for that. If your cheapest really does mean S, N or Q fare classes on CX then status may be an impossible target.

If LHR is one of your destination cities, Sri Lankan would earn more in R class (10+35 each way with MPC) and would likely be less expensive than CX V class. For certain the business cabin would a lot less than CX and the newish A330s are "nice".

Perhaps biased wandering

Fred