I'll be honest, their last record was their worst, and their last great record was Wasting Light. I still wouldn't sleep on Concrete and Gold and Sonic Highways though. There are still some good songs on there too.
As a relatively knew Foo Fighters fan, I have to say that I noticed a drop off in quality as time went on, but I don't think they ever released a bad album. Sure Medicine at Midnight, Sonic Highways, and Concrete and Gold were less consistent than The Colour And The Shape or There Is Nothing Left To Lose, but I still think they were fantastic records with lots of merit to them. Personally my least favourite is Sonic Highways.