This is the $25,000 question. And truth be told, there isn’t a right answer. All frequent flier programs have their pros and cons. However, some might benefit you more than others.
One factor in weighing which one is the best is where you live. Where you live has an impact on which airline you’ll most likely fly on most of the time. If you live in an area where an airlines has a hub, you’ll likely be flying more with that airline. Hubs allow for more non-stop flights to your destination and are likely cheaper than other airlines to the same destination. For instance, if you lived in Atlanta you’ll probably be flying on Delta more because Delta has a hub in Atlanta. Or if you lived in Dallas, you’ll likely be flying on American Airlines more than others.
It is a little trickier if you don’t live in a major metropolitan area that has an airport that serves as a hub for an airline. To make such a decision you need to examine which airline flies into your home airport, to what destinations and how frequently. If airline A flies to two of their hubs once each daily and airline B flies to one of their hubs three times daily, you will most likely have better travel options using airline B because of the frequency of the flights. This is different all areas so if you would like a recommendation, feel free to ask.
Another factor that goes into which frequent flier program is the best, is the airlines’ partners. Almost every airline is a member of an airline alliance that allows earning and redeeming miles on airlines that you don’t have miles on. For example, you can fly on British Airways using your American miles. Alliances expand the places you can travel to without earning miles on multiple airlines, though I do recommend earning miles on airlines in different alliances. More on alliances later.
The last factor that I consider is redemption levels and options. Airlines usually use a zone based redemption chart. That is from North America to Europe is x amount of miles. Others use a distance based chart. For flights from 0-500 miles in distance is x amount of miles. Depending on the amount of miles, zone based charts usually offer the best value. Also, some airlines allow for stopovers or open jaw itineraries for the same amount of miles than a regular round-trip award redemption. An open jaw is where you return to a different airport than you left from. Let’s say NYC – London -Miami. Stopovers are where you stay at a stopover city for 24 hours or more. For example, LAX- London (stop for 3 days) then to Barcelona. There are so many airlines and award options that it wouldn’t make sense for me to write it when someone already did. You can view The Points Guy’s list here.
I hope this helps in choosing your preferred airline to earn miles on. Another couple of things to keep in mind is that you should never fly without earning miles and that you shouldn’t put all your eggs in one basket. Diversify.