Depends on where you want to live in my book. CV has a horrible schedule but it will allow you to fly the venerable 400 if that is what you desire.
CX is great and will allow you to fly the 400 (cruise pilot at first), but you will likely have to live in Hong Kong initially as an SO I believe. I know that a LHR base exists for CX but I don't know if SO newhires can bid it at the beginning. You would get great exposure to Asia obviously.
BA longhaul probably means 777 FO. I have heard it is a great airplane although you will likely be on reserve and fly many Transatlantic or ME flights for the first 3 years or so, you will be able to live in Europe if that is your preference.
I think CV would be great if the schedule were not so bad because the opportunity to fly the 400 worldwide would be great if it weren't so exhausting with so many days gone...
Good luck and let us know which you choose...