check out the Penfed travel rewards American Express card. 5% on airfare purchases, no annual fee, PP select membership, Amex travel benefits, and no foreign exchange fee. This summer it also had 5% back for hotels and dining.

For general purchases I like the FIA Fidelity Amex. If you are loyal to one hotel chain over another then a cobranded card might be worth it; a lot of people seem gaga for the starwood amex.

edited to add: there is probably no single card; you probably need 2 or 3 cards that maximize rewards based on your spending habits. Also, joint credit cards are increasingly rare (speaking about US issuers). More common is making someone an authorized users. You could each get one new rewards card and make each other AUs to capture spending to max. rewards.