It's tough to recommend a good songwriter forum for me, because it's so hard to vet the qualifications of the people who are offering the advice. I suppose it depends on how much practical help you're looking for vs. simply a community to commiserate with.

I always recommend NSAI, but with the disclaimer that I'm on the BOD and some might consider that a bias. The services provided by NSAI are top notch though. Beyond that, while I have personal issues with the founders of Songtown, they do a good job as well. Not a fan of Global Songwriting and don't recommend it. SONA does some of good things as well.

That's a pretty good starting place. Hope that's helpful.