The best actual cash back on unlimited general purchases is probably 2% (Schwab used to do this; Sallie Mae Visa, Capital One Venture or their 2% business card, NASA Credit Union).
5% is basically always either category-limited (Gas, sometimes restaurants) or limited to a trivial (<$5k/yr) spend.
I personally do the American Express Starwood card, since I value Starwood points (1 per $1 in general, and some other bonuses for hotels, gas, etc.) at about $0.025-$0.04. You can also convert SPG points into FF miles at a rate better than unity.
Back in the day, paying for space segment ($200k/mo) on a business credit card meant never having to pay for a hotel suite or airline ticket.
The Chase Amazon card is also 3% on Amazon purchases, which basically means 3% on every mailable product in the world. I think it also gets you 3% on Amazon Web Services purchases...I doubt Dropbox or Netflix pay their bills with that card, but it would be amazing :)
Discover rotates the category of things that give 5% back every few months, which is generally easier to take advantage of than just gas. 5% for everything would be way over the interchange fees, so it'd require some sort of catch.
There is also a limit on the amount applicable to the cashback bonus, per program, and it's usually a trivial amount like $300 (so, $15 cash back per month).
