It really depends where you are going.

I have been using DL for years, feeling almost trapped because of its good TATL coverage - both itself and KL/AF (and notsomuch AZ).
I also liked having CO out of EWR as a backup plan when it was part of skyteam.

But that said, Term 2/3 at JFK is a disaster area and if you don't need the destinations, that's a pretty good incentive to look elsewhere. (Although if I'm not mistaken the new LAX/SFO service is out of T4??)

Anyway CO is strong out of EWR, lots of people like Jetblue if travel is primarily domestic, etc.

Give us a little more info and I'm sure others will chime in with good advice.