I've been playing Dragon Ball Z: Budokai Tenkaichi 3 lately. I've seen all of DBZ and DBGT (and the movies.) I still need to finish Dragon Ball.
I find it rather odd that Frieza would claim to be the strongest being in the universe when there are entities out there that can easily crush him. Broly and Bojack are just some examples, seeing as they're both powerful enough to beat a standard Super Saiyan.
I know Cooler has that 5th transformation to tell that he's stronger but I think at least in the form we saw King Cold in the show, Frieza at the time was stronger. Perhaps he'd be stronger if he were allowed to show his other transformations, but I don't think 2nd-form King Cold is as strong as full power Final Form Frieza or Mecha Frieza at the time.Trunks killed King Cold before he could even think of transforming, so we'll never know for sure.
Well Frieza never knew of Broly and Bojack were sealed away. Still, Frieza is a liar because King Cold and Cooler were stronger.