Good question. I think I would go with these three:
1) AMEX Starwood - One of the best cards in the industry right now.
2) Diners Club - As long as they don't cut off too many benefits. With Mastercard acceptance, it will make it a lot easier to earn Club Rewards points.
3) Discover Gas card - 5% cashback at all gas stations that can be redeemed in $20 increments, great in this day of colossal gas prices.