Longest in terms of playing time? (That's how it reads, as you've worded it) Or longest in terms of development time? (This is what I suspect you're going for).
LOL! Did you really bump a thread after less than three hours? That's not bumping, bro. That's "why is no one answering this question that is important only to me?"
I agree, Forgotten Memories has been in development forever! But once it comes out, it's going to be sweet
Galactic Keep would get my vote, although there's probably others that were announced earlier in genres that I don't really follow.