Depends
It depends on what you want to do and where you want to go. On the North Side, there's Hicks and Northwest Regional. Just to the North of downtown, there's Meacham (KFTW). To the South there's Spinks (KFWS) and Sycamore (9F9) (Sycamore's a little rough and narrow), and as you go East, there's Arlington (KGKY), Grand Prairie (KGPM), and Dallas Executive (Redbird). Generally, the ground traffic's better on the South side. Most of these airports, but not all, have full service like gas, rental cars, loan cars, maintenance, etc. If you want easy access to the entire metroplex, I'd use Arlington or Grand Prairie. If it's just Fort Worth, I'd go for Meacham or Spinks, or, if I needed to be on the North side, Hicks or NW Regional.