There's a long discussion thread on this topic in the SPG forum because for many the answer used to be the SPG Amex, but with that card being devalued this summer there are a lot of people thinking about what the next best option should be.
But the answer really depends on your personal spending habits and redemption goals. It only really makes sense to branch out into Amex Membership Rewards or Citi Thank You Points if you have a lot of spend -- like if you have a lot of reimbursed business expenses or otherwise get creative (
) with your credit card spending. If you are just putting personal expenses on the card, for most people you'd just end up spreading yourself too thin across multiple programs and won't have enough points earned in any one program to do anything valuable with them.
Not to mention that there is a lot of overlap in bonus categories between the CSR and the other cards you mention so you're not opening up many new channels for earning bonuses.
An airline card could be valuable if you have use for the specific benefits of such a card -- for example Delta's credit cards offer certain accelerators toward earning elite status that could be valuable if you are within striking distance of Delta elite status. Or it can be valuable to hold an airline card, even if you don't spend much on it, for benefits like a free checked bag if you don't have elite status. But if you're not getting anything specific out of it, then there's not much reason to use an airline card. In general you'd get a higher return on general spend using the CFU than any of the major airline cards.