I personally use SkyMiles, since Delta is my preferred carrier, and I can get upgrades on DL
with my status there - if I had status on CO
, or any other airline, I could get preferred coach seating, but would not get the upgrade.
In terms of making elite status, Czech is the best, only need 15 segments to make OK
Plus Silver. DL
/CO/NW are all very much the same, needing 25000 miles (or 30 segments in NW
's case) to make Silver, I don't know KL
, or AM
's requirements, but they are similiar.