FMCKJ is generally viewed as the most liquid of the preferreds. 240,000,000 shares were issued circa late 2007, I believe.
Thus, FMCKJ is pretty easy to buy, not a wide bid-ask spread, etc. But its a bit price-y so FLCKI and FLCKL might be options if you are in a hurry to build a basic position.
There was a stockholder suit re FMCKJ but I believe that that suit failed (FHFA as conservator told the Court that Freddie Mac had met its disclosure requirements, when the FMCKJ shares were issued). But, I'm not a lawyer.
The $50s tend to be cheaper but harder to buy. Not to say that you can't build positions in them, but one would need to be patient.