The best prerecorded cassettes I ever encountered were the CrO2 tapes Advent put out to show off the potential of their cassette deck. I encountered them when I worked at a stereo shop in SoCal in the mid-'70s, as we were an authorized Advent (and Revox, Nakamichi, and Tandberg) dealer.
I got my comeuppance when I tried to dub some of these (mostly classical) to my Tandberg reel-to-reel. I could usually dub LPs to 3-3/4 ips with plenty of headroom, but with these cassettes, I not only had to switch to 7-1/2 ips, it was quite difficult to dial in a recording level that didn't max out the meters and (presumably) oversaturate the tape.
I got my comeuppance when I tried to dub some of these (mostly classical) to my Tandberg reel-to-reel. I could usually dub LPs to 3-3/4 ips with plenty of headroom, but with these cassettes, I not only had to switch to 7-1/2 ips, it was quite difficult to dial in a recording level that didn't max out the meters and (presumably) oversaturate the tape.