We are Delta fans. In the past 11 years we have also flown Sunwing (a Canadian airline), AirTran (which i believe was purchased by Southwest?), Jetblue, and Frontier.
We will not fly any airline with open seating, so Southwest is not an option for us. DH, at 6'4 and 300+ pounds, absolutely needs an aisle seat and more space/legroom so that affects our decisions as well.
Delta has been tops in service, by a long shot. We have the Skymiles platinum Amex so we able to earn free flights every year, just by using the card for our normal household purchases.
We also liked Jetblue, but in 2017 we moved to an area where they dont offer nonstop flights to MCO. Nonstops are also a must for us. Even while we enjoyed our Jetblue flights, it would be hard for us to go with any other carrier besides Delta now that we have become used to their multiple schedule options and excellent service.