I'll give you the quick answer. If your travel is domestic coach travel then you won't do better than a flat 2% cashback card, with some exceptions (basically the average domestic airline award ticket using real miles is worth less than 2% but occasionally more, but you have far less flexibility compared to a 2% cashback card, and for many people it's frustrating if you don't understand how award tickets work).

People that swear by the "premium" cards like the Chase Sapphire Preferred, Amex cards, and Starwood card are redeeming for premium awards like international business class flights and middle to high end hotels. You can get far above 2% value here, but only if this is what you want to redeem for, otherwise you get very little value.

Generally favor a flexible card like the Sapphire Preferred or amex card over an airline specific card unless you fly one airline all the time and would benefit from the additional perks like free checked bags.

As for Amex, depends on where you live and where you shop, I live in Los Angeles and can use my Amex for 97% of my spending.

People will give you some good ideas here but there's no standard comparison between cards because some are better for some people versus others. The general rule is unless you KNOW you can redeem flexible points/miles for travel at a greater rate than 2% (and this means you've looked at booking award flights or hotel stays and understand how many points you need etc) then stick with a flat 2% card and don't worry about anything. But for those that like premium award travel and know how to do it, a 2% card would be leaving money on the table.