I use the rapidfire for akimbo, the jitter for SPAS-12. I do use rapidfire for the FAL with suppressor but I only use that combination maybe 30% of the time... I've started using the full auto Enfield with rapidfire turned on to actually slow down the firing rate to conserve ammo.
So to answer your question in my opinion FAL with rapidfire is no longer the best gun in COD:BO. I think it was much more powerful in COD:MW2. But I may not be running the correct perks to get the most power out of it. When I do use it I prefer the suppressor version, I don't like to give off my position since I run out of ammo somewhat quickly with FAL and are reloading frequently. (don't want to fire, give off my position, run out of ammo, reload and get killed while reloading)