My current usage, in order of preference:

Hilton HHonors AMEX - 3 points/dollar on everything, 6 points/dollar on Hilton, internet, cell phone, and drugstores. I will probably upgrade to the new Surpass card to get Diamond for $40k in spend, though that is a bit of a stretch, since I have other spending on other cards like ...

Citi mtvU (similar to new Citi Forward card) - 5 TYP/dollar on restaurants, movies, and bookstores

Platinum AMEX - 2 MR points/dollar on gas and groceries (I was basically not using this card at all except for lounge access and Fine Hotels and Resorts before this new year-long promo)

Diners Club - 1 point/dollar on rental cars (50% bonus on BA transfers usually offered annually too)

Discover Card - Whatever they give 5% back on up to the limit

I don't use my Platinum AMEX or Diners Club much, but I like being in both programs and keeping points there for top-ups and promotions (i.e., promotions where you need x transactions from different partners, which both US and DL have run recently), they come in very handy.