Seat comfort for the flight is probably more important as a consideration, and last time I checked JL/AN only had the good seats to LHR and not their other European destinations. All 3 airports are OK, but FRA is quite efficient and likely to have shorter connections than LHR (on the other hand the longer flight to LHR gives more time for sleeping on the plane). The easiest airports for transfers are ZRH and BRU but I really think the airport choice is a minor consideration compared to flight schedule, connections and airplane configuration. CDG does have a terrible shortage of gates due to the terminal collapse and I try to avoid it, but it is functional.