For me the bestl deck is one that performs well and is reliable, such as the Pioneer CT-S88R that I now own. It's well reviewed upper mid-line deck from the late 80s and hardly state-of-art, even for 1986, but it works great and satisfies me completely. And, no, I am not claiming for it "holy grail" status.
In my day I've owned many decks, including seven middle range Nakamichi models but the only 'holy grail' machine I've owned is a Pioneer CT-F1250, which is incredible, and a Tandberg (I forget the model) which was spectacular but very unreliable. Another friend owned a stunning Revox cassette deck but I never had the opportunity to play with it. It certainly was a attractive machine.
I've known a couple of guys who owned either a Nakamichi 700 or 1000 model and those decks impressed me big time. Not only were they incredible tape decks, they were B E A U T I F U L.