I once sent some cards to TFA (The Final Authority). They were a little generous with the grades, but not terrible. Thank goodness I never submitted cards to some of the other companies mentioned here. I'll take other peoples' word for it instead of getting burned first hand! I think my vote for worst would go to ANY company that uses a screw down holder! If anyone with a Phillip's head screwdriver can break into your slab without effort - you've got problems!
Don't worry, I didn't sentence any great cards to a prison of an off brand slab. Actually, it was either 1999 or 2000 when TFA was brand new and they were charging $5.00 a card. I was fairly new to grading, and I gave them a try. That was the first and last time I ever submitted to a company other than the "big three". I learned my lesson.
While Beckett is clearly not the worst grading service, I am always cautious about buying vintage cards in Beckett holders as I have had SGC send two Becketts back to me with the comment "evidence of trimming'. But I have also had some BGS 8 cards cross to SGC 88 including my 1971 Clemente card.