best credit card that accrues frequent flyer miles

Just remember that most credit cards that accrue frequent flier miles have an annual fee. Therefore you have to question whether or not it is worth having a credit card that you have to pay between $50-$75 a year to keep. Capital One offers a credit card which allows you to accumulate miles and there are no black-out dates. However you have to read to see if there is a limit on the number of miles you can earn per year. Also with so many people accruing frequent flier miles on non-flying related charges, it is harder to actually be able to cash in your miles for a free ticket.
Also, look for a card that has no annual fee. If you have to pay $50 to earn the miles, that offsets some of the value of the miles.

As a side note - I travel a lot for business, and in general I have found that hotel point CCs offer better value per dollar charged and generally are easier to redeem. For example, I use the Hilton HHonors card. I get 2 points for every dollar charged to the card, so for $20,000 I end up with 40,000 points, or enough for about $400 in free room nights that I have never had a problem using.
By comparison, USAir's card would earn me 20,000 miles for those purchases, which is not even enough for a round trip ticket on their cheapest option (and good luck finding a mileage saver seat anywhere popular). You would need to spend $50,000 on the card to get a round trip ticket inside the us, average value around $400 (plus you pay a fee to redeem miles at most airlines now)
My suggestion is to find a card that you can earn points or receive cash back. I have a Visa credit card (Freedom card) from Chase where you can earn 3 points for every dollar on the top 3 categories you spend on such as groceries and gas. Once you accumulate 5,000 points, you can cash that in for $50. There is no annual fee associated with this. Also if you apply right now, Chase will give you 5,000 points after your first purchase. Therefore you will get $50 after making 1 purchase.

And in terms of true value, cashback cards can be the best deal, IF you can find one without an annual fee.