I loved Manuwai when he came out of Hawaii a few years ago. I'd love to get him now. Really, anyone of the top FA guards would work here: ERic Steinbach, Kris Dielman, Jake Scott, Sean Mahan, and Cooper Carlisle.
If the Falcons picked up Manuwai, Gibbs would probably make him slim back down some, probably closer to where he was at when he first came into the league (305). I'd be really disappointed if the Falcons walked away from FA without signing one of these guys.
Particularly Steinbach and Dielman would be big upgrades at LG.
I don't know if there is 1 FA that I'd put above all the rest as the guy I'd want most, but I do have a short list of guys that I really like the way they play and think they'd do wonders in Atlanta (no order):
1. Ladell Betts. I've always considered him one of the most underrated backup RBs in the NFL, and he showed that he's one fo the best today.
2. Daniel Graham. To me a duplicate of Crumpler that is underused in NE.
3. Rian Lindell - HIts 90% of his kicks in Buffalo, imagine what he'd do in Atlanta.
4. Ken Hamlin - great safety, but probably wouldn't pass our filter.
5. Nate Clements - a bit overhyped, but then again who isn't these days?
6. Rod Hood - The next Al Harris.
7. Dwight Freeney - To I have to say anything?
8. Justin Smith/Jared Allen - Both are younger versions of Kerney but stronger vs. the run.
9. Any of those OGs I mentioned above.