Avoid CO! Upgrades are a game in and of itself. Do a search on "HoKeY" and "BendForward" or "BF" on the CO forum.

On a practical note, also avoid CO because they send their infamous B757s across the Atlantic. Also, connecting in EWR isn't fun, especially dealing with the TSA after exiting customs.

Assuming that you're a European resident, I have nothing to say about KLM or BA's ff programs.