The best airline miles card is Discover this card has no restrictions on when you could use them plus you could you use your miles for other free stuff, check out www.fastcreditcardapprovals.com under reward cards.
There are quite a few factors to determine which card would be best for you (mileage expiration, lowest miles for trip, etc.). This site gives you some ideas of how to choose the right card for you:
If you're only going for miles and live near a major hub airport, it would be best to get a card offered by an airline. They often have special deals; for example, I think United still has their offer of 30,000 miles after the first $250 spent, one mile for each $1 after that, and a 20,000 bonus after the first year.