If you scroll down, you will see that K-Fraud's so-called "album", "Playing With Fire", quite naturally has the lowest ever recorded Metascore on Metacritic.com, which collects ratings from reviews from pretty much everywhere and lumps them together. It has scored a 15/100, and I think they were being generous. The second worst rated album (Hefty Fine by Bloodhound Gang) is 13 points higher.
Mind you this only includes albums released in the last few years.
Here's the detailed link for the album:
I love how the BEST review is from Billboard, who managed to state that "In general, Federline enunciates well" as their big concession.