Right now I'm using American Express. I was using Visa/Mastercard for quite some time, but lately and after reviewing what it offers, American Express has been too good to pass up. What I found to be of maximum value, was its deals for shopping at selected vendors and you get three times the point for every dollar.
On a side note, like most of us, if I use a number a lot, I tend to remember it, and that goes for credit card numbers... comes in real handy when you don't have the card on your person. But switching to AE was ironic because it's not a typical 16-digit number, in fact, it's not even sixteen digits, but fifteen - but in an odd sequence(4-6-5), which I found to be interesting since every credit card I've ever used(even debit cards) have been 16 digits in groups of four numbers. :)