- What is more important? Price or schedule?
- Are non-stop flights important? Are you willing to make a connection or two?
In the aviation business airlines fly to and from their hubs. i.e. BA flies to/from Heathrow and Gatwick, LH flies to/from FRA/MUC. AF to/from CDG/ORY. BA do not fly between FRA and CDG and LH don't fly between LGW and ORY. So to get the widest choice one would stick with their national carrier, if they live close to a hub.
Of course Ryanair and easyJet are the exception to this rule (wizz to a lessor extent). They fly between many non-hub cities. Of course the LCC model means no connecting flights, sometimes obscure airports and non-refundable fares (depending on the airline, of course).
I'd fly any European airline and vary my expectations accordingly.