Most credit cards have an upper limit on how many points or miles you can earn in a year (usually around 100,000). So far as I know, the American Express Starwood card has no such limit. Every 20,000 points convert to 25,000 Delta Airlines miles. (The conversion rate to United is not so good.) You may read about this card on my website below on the Credit Cards > Free Cards page. It's the first one listed, as it is most everyone's favorite card for acquiring miles.

I believe that most or all of the American Express Membership Rewards cards have no upper limits on the number of points that can be earned per year. The Premier Rewards, Gold Business, and (if you like extra benefits) the Platinum cards may be of interest to you. All have mega up front bonuses. They are described on my website below on the Annual Fee Cards page in the American Express section near the bottom of the page. Amex especially specializes in cards of interest to businesses with its Open program.

You might also look at the Stock and Bond brokers offers on my Finance page for still more United, Continental (soon to be United), or Delta miles, or Amex points.